WASHINGTON (AP) — Defying Russian warnings against U.S. military strikes in Syria, President Donald Trump said Wednesday that missiles “will be coming” in response to Syria’s suspected chemical attack that killed at least 40 people.

“Russia vows to shoot down any and all missiles fired at Syria,” Trump tweeted. “Get ready Russia, because they will be coming, nice and new and ‘smart!’ You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”

Defense Secretary Jim Mattis, however, indicated that evidence of what happened in Saturday’s suspected attack near Damascus is still being studied. At a photo-taking session during a meeting with his Dutch counterpart, Mattis was asked by a reporter whether he had seen enough evidence to blame the Syrian government.

“We’re still assessing the intelligence, ourselves and our allies,” Mattis said. “We’re still working on this.”

Asked whether the U.S. military was ready to conduct an attack in Syria if ordered, Mattis replied, “We stand ready to provide military options if they’re appropriate, as the president determines.”

Trump did not detail what a strike would look like, or whether these would be U.S. missiles. U.S. officials have been consulting with global allies on a possible joint military response to Syria’s alleged poison gas attack. Trump canceled a foreign trip in order to manage a crisis that is testing his vow to stand up to Syrian President Bashar Assad.

Syria’s Foreign Ministry said Wednesday that his threats are “reckless” and endanger international peace and security.

Trump’s tweet also drew a quick response from Russia’s Foreign Ministry. Spokeswoman Maria Zakharova said on Facebook that “smart” missiles would destroy any evidence of a chemical weapons attack.

The term “smart” missile dates to the introduction decades ago of weapons with advanced guidance systems — for example, using GPS — to achieve greater precision in targeting. The term was meant to contrast with “dumb” bombs lacking such technology. Virtually all U.S. missiles now have advanced guidance systems.

Russian lawmakers have warned the United States that Moscow would view an airstrike on Syria as a war crime and that it could trigger a direct military clash between the two former Cold War adversaries. Russia’s ambassador to Lebanon said any missiles fired at Syria would be shot down and the launching sites targeted — a stark warning of a potential major confrontation.

Shortly after warning that missiles were coming, Trump tweeted that the U.S. “relationship with Russia is worse now than it has ever been, and that includes the Cold War.” Then, striking a more conciliatory tone, he said: “There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?”

Trump’s administration has sought to show toughness on Russia, with a series of economic and diplomatic actions, including new sanctions last week against government officials and oligarchs. Trump has largely avoided criticizing Russian President Vladimir Putin by name, though he singled him out in a tweet over the weekend for supporting Assad.

In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 election campaign. During one speech, he said, “We must as a nation be more unpredictable. We are totally predictable. We tell everything. We’re sending troops. We tell them. We’re sending something else. We have a news conference. We have to be unpredictable.”

The U.S., France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, U.S. officials have said. A joint military operation, possibly with France rather than the U.S. in the lead, could send a message of international unity about enforcing the prohibitions on chemical weapons and counter Syria’s political and military support from Russia and Iran.

French President Emmanuel Macron said his nation, the U.S. and Britain will decide how to respond. He called for a “strong and joint response” to the attack in the Syrian town of Douma on Saturday, which Syrian activists and rescuers say killed at least 40 people. The Syrian government denies responsibility.

The French president does not need parliamentary permission to launch a military operation. France is already involved in the U.S.-led coalition created in 2014 to fight the Islamic State group in Syria and Iraq. Multiple IS terror attacks have targeted France, including one last month.

Trump suggested Monday he had little doubt that Syrian government forces were to blame for what he said was a chemical attack, but neither he nor other administration officials have produced hard evidence. This is in contrast to an incident one year ago in which U.S. intelligence agencies had video and other evidence of certain aspects of the actual attack, which involved the use of sarin gas. Trump responded then by launching Navy cruise missiles at a Syrian airfield.

One official said the U.S., France and Britain were considering military options that would be more extensive than the punitive, one-day strike last April. The three countries are discussing a range of options, including preventing Assad from conducting future attacks by striking military capabilities involved in carrying out such attack, the official said.

That official was not authorized to speak publicly and commented only on condition of anonymity.

Macron, asked whether France would take military action, said his country would continue discussing technical and strategic information with U.S. and British allies and “in the coming days we will announce our decision.” He said any action would “target chemical weapons” stocks. Under a 2013 agreement for which Russia was a guarantor, Syria was to have eliminated all its chemical weapons, but it has used chlorine and perhaps other chemicals since then.

Trump spoke by phone with British Prime Minister Theresa May. A British government statement said the two agreed the attack in Syria was “utterly reprehensible” and the international community must respond “to uphold the worldwide prohibition on the use of chemical weapons.” Trump met at the White House with the emir of Qatar, Sheikh Tamim bin Hamad Al Thani, who told reporters that he and Trump “see eye to eye” on Syria.

Qatar hosts the United States’ main air operations center for the Middle East, which would coordinate any American air attack in Syria.

A watchdog agency, the Organization for the Prohibition of Chemical Weapons, announced that it would “shortly” send a fact-finding mission to Douma, after receiving a request from the Syrian government and its Russian backers to investigate the allegations. It was not immediately clear whether that visit might affect any U.S. or allied military action.

The Russian military, which has troops in Syria, said on Monday that its officers had visited the site of the alleged attack and found no evidence to back up reports of poison gas being used.

President Donald Trump waits at the main entrance to the West Wing for the arrival of the Emir of Qatar Sheikh Tamim bin Hamad al-Thani, at the White House in Washington, Tuesday, April 10, 2018. (AP Photo/Manuel Balce Ceneta)