WASHINGTON (AP) — Secretary of State John Kerry on Monday outlined the clearest justification yet for U.S. military action in Syria, saying there was “undeniable” evidence of a large-scale chemical weapons attack, with intelligence strongly signaling that Bashar Assad’s regime was responsible.

Kerry, speaking to reporters at the State Department, said last week’s attack “should shock the conscience” of the world.

“The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable and — despite the excuses and equivocations that some have manufactured — it is undeniable,” said Kerry, the highest-ranking U.S. official to confirm the attack in the Damascus suburbs that activists say killed hundreds of people.

“This international norm cannot be violated without consequences,” he added.

Officials said President Barack Obama has not decided how to respond to the use of deadly gases, a move the White House said last year would cross a “red line.” But the U.S., along with allies in Europe, appeared to be laying the groundwork for the most aggressive response since Syria’s civil war began more than two years ago.

Two administration officials said the U.S. was expected to make public a more formal determination of chemical weapons use on Tuesday, with an announcement of Obama’s response likely to follow quickly. The officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.

The international community appeared to be considering action that would punish Assad for deploying deadly gases, not sweeping measures aimed at ousting the Syrian leader or strengthening rebel forces. The focus of the internal debate underscores the scant international appetite for a large-scale deployment of forces in Syria and the limited number of other options that could significantly change the trajectory of the conflict.

“We continue to believe that there’s no military solution here that’s good for the Syrian people, and that the best path forward is a political solution,” State Department spokeswoman Marie Harf said. “This is about the violation of an international norm against the use of chemical weapons and how we should respond to that. “

The Obama administration was moving ahead even as a United Nations team already on the ground in Syria collected evidence from last week’s attack. The U.S. said Syria’s delay in giving the inspectors access rendered their investigation meaningless and officials said the administration had its own intelligence confirming chemical weapons use.

“What is before us today is real and it is compelling,” Kerry said. “Our understanding of what has already happened in Syria is grounded in facts.”

The U.S. assessment is based in part on the number of reported victims, the symptoms of those injured or killed and witness accounts. Administration officials said the U.S. had additional intelligence confirming chemical weapons use and planned to make it public in the coming days.

Officials stopped short of unequivocally stating that Assad’s government was behind the attack. But they said there was “very little doubt” that it originated with the regime, noting that Syria’s rebel forces do not appear to have access to the country’s chemical weapons stockpile.

Assad has denied launching a chemical attack. The U.N. team came under sniper fire Monday as it traveled to the site of the Aug. 21 attack.

It’s unclear whether Obama would seek authority from the U.N. or Congress before using force. The president has spoken frequently about his preference for taking military action only with international backing, but it is likely Russia and China would block U.S. efforts to authorize action through the U.N. Security Council.

Kerry on Monday made several veiled warnings to Russia, which has propped up Assad’s regime, blocked action against Syria at the U.N., and disputed evidence of the government’s chemical weapons use.

“Anyone who can claim that an attack of this staggering scale can be contrived or fabricated needs to check their conscience and their own moral compass,” he said.

British Prime Minister David Cameron, who like Kerry cut short his vacation because of the attack, spoke Monday with Russian President Vladimir Putin to outline the evidence of chemical weapons use by Assad’s regime.

Cameron’s office also said the British government would decide on Tuesday whether the timetable for the international response means it will be necessary to recall lawmakers to Parliament before their scheduled return next week. That decision could offer the clearest indication of how quickly the U.S. and allies plan to respond.

Late Monday, the State Department said it is postponing a meeting with Russian diplomats on Syria this week. The meeting at The Hague was about setting up an international conference to find a political resolution to the Syrian crisis. A senior State Department official said Monday the meeting between Undersecretary Wendy Sherman and U.S. Ambassador to Syria Robert Ford with their Russian counterparts was postponed because of the ongoing U.S. review about alleged use of chemical weapons in Syria.

The U.S. official said the meeting will be rescheduled because a political solution is still needed in Syria. The official was not authorized to publicly confirm the changes and spoke on condition of anonymity.

More than 100,000 people have died in clashes between forces loyal to Assad and rebels trying to oust him from power over the past two and a half years. While Obama has repeatedly called for Assad to leave power, he has resisted calls for a robust U.S. intervention, and has largely limited American assistance to humanitarian aid. The president said last year that chemical weapons use would cross a “red line” and would likely change his calculus in deciding on a U.S. response.

Last week’s attack in the Damascus suburbs is a challenge to Obama’s credibility. He took little action after Assad used chemical weapons on a small scale earlier this year and risks signaling to countries like Iran that his administration does not follow through on its warnings.

Syrian activists say the Aug. 21 attack killed hundreds; the group Doctors Without Borders put the death toll at 355 people.

The most likely U.S. military action would be to launch Tomahawk cruise missiles off U.S. warships in the Mediterranean. The Navy last week moved a fourth destroyer into the eastern Mediterranean.

Officials said it was likely the targets would be tied to the regime’s ability to launch chemical weapons attacks. Possible targets would include weapons arsenals, command and control centers, radar and communications facilities, and other military headquarters. Less likely was a strike on a chemical weapons site because of the risk of releasing toxic gases.

Military experts and U.S. officials on Monday said that the precision strikes would probably come during the night and target key military sites.

The president has ruled out putting American troops on the ground in Syria and officials say they also are not considering setting up a unilateral no-fly zone.

On Capitol Hill, bipartisan support for a military response appeared to be building.

Rep. Adam Schiff, a California Democrat on the House Intelligence Committee, backed the idea of targeted strikes against key Syrian infrastructure, saying it would both be a deterrent to future use and carry “less risk of drawing us in further, or spreading the conflict.”

Republican Sen. Bob Corker of Tennessee said he would support “a surgical, proportional strike” against the Assad regime, but called on the administration to seek congressional authorization for such actions.

In another sign that a U.S. response may be fast-approaching, the White House on Monday was reaching out to congressional leaders. A spokesman for House Speaker John Boehner said the Ohio Republican had “preliminary communication” with White House officials about the situation in Syria and a potential American response.

Kerry made a series of private calls Monday to senior lawmakers to talk about the situation in Syria, congressional aides said. Kerry told the senators and congressmen that officials are sure the Assad regime has used chemical weapons, the aides said. But he offered no indication on plans for responses, saying the U.S. was still coordinating with allies.

The aides spoke anonymously because they weren’t authorized to provide details of the private discussions

Syria was also the subject of a call Monday between Obama and Australian Prime Minister Kevin Rudd. The White House said the two leaders discussed possible responses by the international community to the use of chemical weapons near Damascus. And as part of ongoing consultations, Obama National Security Adviser Susan Rice met with a delegation of top Israeli officials. The White House said topics covered were developments in Iran, Egypt, Syria and other regional security issues.

It’s unlikely that the U.S. would launch a strike against Syria while the United Nations team is still in the country. The administration may also try to time any strike around Obama’s travel schedule — he’s due to hold meetings in Sweden and Russia next week — in order to avoid having the commander in chief abroad when the U.S. launches military action.

U.N. Secretary-General Ban Ki-moon on Monday countered the U.S. claim that the investigation at the site of last week’s attack was too little, too late.

“Despite the passage of a number of days, the secretary-general is confident that the team will be able to obtain and analyze evidence relevant for its investigation of the August 21 incident,” U.N. spokesman Farhan Haq said in New York