TAIPEI, Taiwan — Five U.S. lawmakers met with Taiwanese President Tsai Ing-wen on Friday in a surprise one-day visit intended to reaffirm the United States’ “rock solid” support for the self-governing island.

The bipartisan group of lawmakers from the U.S. House of Representatives arrived in Taiwan on Thursday night and planned to meet with senior leaders including Tsai, said the American Institute in Taiwan, the de facto U.S. embassy. No further details were provided about their itinerary.

The visit comes as tensions between Taiwan and China have risen to their highest level in decades. Taiwan has been self-ruled since the two sides split during a civil war in 1949, but China considers the island part of its own territory.

China was quick to condemn the trip when news first broke Thursday night.

“When news of our trip broke yesterday, my office received a blunt message from the Chinese Embassy, telling me to call off the trip,” Representative Elissa Slotkin, a Michigan Democrat who is part of the delegation, wrote on Twitter.

In Beijing, Chinese Foreign Ministry spokesperson Zhao Lijian condemned the visit, calling it a violation of the “one-China principle” under which Taiwan is considered part of China.

“That individual U.S. politicians wantonly challenge the one-China principle and embolden the ‘Taiwan independence’ forces has aroused the strong indignation of 1.4 billion Chinese people,” Zhao said. He added that unification of Taiwan and China is an “unstoppable historical trend.”

Representatives Mark Takano, D-Calif., Colin Allred, D-Texas., Sara Jacobs, D-Calif., and Nancy Mace, R-S.C., are also part of the delegation.

“We are here in Taiwan this week to remind our partners and allies, after two trying years that we’ve endured, that our commitment and shared responsibility for a free and secure Indo-Pacific region remain stronger than ever,” Takano said.

Takano added that the U.S. relationship with Taiwan is “rock solid and has remained steadfast as the ties between us have deepened.”

Tsai, who welcomed the lawmakers at the Presidential Office in Taipei, noted the two sides’ cooperation in veterans’ affairs, economic issues and trade while reiterating the island’s close alignment with the U.S.

“Taiwan will continue to step up cooperation with the United States in order to uphold our shared values of freedom and democracy and to ensure peace and stability in the region,” Tsai said.

The visit is the third by U.S. lawmakers to Taiwan this year and comes just a few weeks after a group of six Republican members of Congress visited the island. That delegation met with President Tsai, National Security Secretary General Wellington Koo and Foreign Minister Joseph Wu, among others.

In June, three members of Congress flew to Taiwan to donate badly needed vaccines at a time when the island was struggling to get enough.

The Biden administration has also invited Taiwan to a Summit for Democracy next month, a move that drew a sharp rebuke from China.

In the Solomon Islands this week, anti-government rioters held protests and looted parts of the capital city in part over complaints about a 2019 decision to switch diplomatic recognition from Taiwan to China. China has been on a campaign to poach Taiwan’s remaining diplomatic allies.