The drills took place near the Taiwan Strait, the 110-mile divide between Taiwan and China. For decades, China has asserted that Taiwan is its territory, though the two split in 1949 during the Chinese Civil War.

Though the U.S. has kept its relations with Taiwan informal, the relationship continues to be strong. AP reported that the delegation arrived in Taipei on Tuesday, though no other details were immediately available.

In the AP report, Chinese Foreign Ministry spokesperson Wang Wenbin said China considers the visit a “serious violation” of the commitment to not have formal relations with Taiwan.

“All risky and provocative actions against China’s reunification is like an ant trying to topple over a giant tree and is doomed to fail,” Wang said.

The drills were led by the PLA’s Eastern Theater Command. In an announcement on Tuesday, the Chinese Defense Ministry called the drills a “necessary measure to safeguard national sovereignty.”

The ministry said the drills are a “joint war preparedness patrol” as a result of “seriously incorrect words and actions of relevant countries over the issue of Taiwan.”

For more reporting from the Associated Press, see below.

A Chinese Defense Ministry statement from an unidentified spokesperson strongly condemned the visit, saying “no one should underestimate the firm determination of the People’s Liberation Army to safeguard the Chinese people’s national sovereignty and territorial integrity.”

China-Taiwan relations have grown increasingly tense under Taiwan’s independence-leaning President Tsai Ing-wen.

During China’s National Day weekend in early October, China dispatched 149 military aircraft southwest of Taiwan in strike group formations, causing Taiwan to scramble aircraft and activate its air defense missile systems. Taiwan’s Defense Ministry said this week such tactics were aimed at wearing down the island’s defenses and degrading morale.

In Washington, Pentagon spokesperson John Kirby said congressional visits to Taiwan “are relatively common and in keeping with U.S. obligations under the Taiwan Relations Act,” which requires the U.S. government to ensure Taiwan has the ability to defend itself and regard threats to the island as matters of “grave concern.”

The delegation arrived in Taipei on Tuesday evening aboard a C-40 Clipper jet, which departed soon afterward, according to Taiwan’s official Central News Agency. Kirby said traveling on a U.S. military jet was customary for such delegations.

Taiwanese Foreign Ministry spokesperson Joanne Ou said the ministry had worked with the American Institute in Taiwan, which is the de facto U.S. embassy, on arrangements for the visit but gave no details. She said further information would be released at the “appropriate time.”

Although the U.S. switched diplomatic ties from Taipei to Beijing in 1979, it retains strong informal political and military relations with Taiwan. As a vibrant democracy, Taiwan also enjoys strong bipartisan support in Congress and the U.S. government has been boosting relations through high-level visits and military sales.