Chinese bombers and warships conducted drills around Taiwan on Monday, the latest military maneuvers near the self-ruled island that a senior U.S. official denounced as "coercion" and a threat to stability in the region.
The United States has no formal ties with Taiwan but is bound by law to help provide the island with the means to defend itself and is its main source of arms.
China has stepped up pressure on Taiwan, whose President Tsai Ing-wen Beijing suspects of pushing for the island's formal independence, a red line for China which has never renounced the use of force to bring Taiwan under its control.
"Any attempt to influence Taiwan through threats or coercion, we believe, destabilizes the region and threatens stability in the Taiwan Strait," James Moriarty, chairman of the American Institute in Taiwan, said at a ceremony to mark the last four decades of U.S.-Taiwan relations.
China's People's Liberation Army said its warships, bombers and reconnaissance aircraft had conducted "necessary drills" around Taiwan on Monday, though it described them as routine.
China has repeatedly carried out what it calls "island encirclement patrols" in the past few years.
Taiwan scrambled jets and ships to monitor the Chinese forces, its defense ministry said, accusing Beijing of "trying to change the status quo of the Taiwan Strait."
Moriarty said "flying fighter jets and bombers around the island, presumably in connection with what we are doing this afternoon, certainly does not help at all.
"It hurts stability. It damages the cross strait relationship. It damages any attempt by China to win the hearts and minds of Taiwan people," he told reporters at the institute's new $256 million facility, the de facto U.S. embassy in Taiwan.
A delegation led by former U.S. speaker of the House of Representatives, Paul Ryan, was in Taipei to mark 40 years since the signing of the Taiwan Relations Act, which governs U.S.-Taiwan relations, and to reaffirm Washington's commitment.
"We couldn't ask for a better friend than Taiwan. Taiwan is a democratic success story, a reliable partner, and a force for good in the world," Ryan said. "We want the rest of the world to be more like Taiwan."
Tsai, who says she wants to maintain the status quo with China but will defend Taiwan's security and democracy, said at the ceremony that China has been ramping up military threats against Taiwan.
The visit by U.S. officials comes just weeks after Tsai said the United States was responding positively to Taipei's requests for new arms sales to bolster its defenses in the face of growing pressure from China.
Last month, Washington sent Navy and Coast Guard ships through the narrow strait separating the island from the mainland, part of an increase in the frequency of U.S. movement through the strategic waterway to show support for Taipei.