China sent warships and aircraft near Taiwan for a second day on Friday and said the island remains its "inseparable part", after President Tsai Ing-wen angered Beijing by meeting with U.S. House Speaker Kevin McCarthy.
Three Chinese warships sailed in waters surrounding the self-ruled island, while a fighter jet and an anti-submarine helicopter also crossed Taiwan's air defence identification zone (ADIZ), the Defence Ministry said in Taipei.
Ms. Tsai returned on Friday after visiting Taiwan's dwindling band of official diplomatic allies in Latin America, with two U.S. stopovers that included meetings with Mr. McCarthy and other lawmakers.
"We let the international community see that Taiwan is more united when facing pressure and threats," she told reporters, describing her trip as a success.
"We will never yield to suppression and we will not stop interacting with the world because of any hindrance."
Hours before her meeting with Mr. McCarthy in Los Angeles on Wednesday, China sent its Shandong aircraft carrier through Taiwan's southeastern waters on its way to the western Pacific.
Beijing said earlier on Friday that "Taiwan is an inseparable part of China", after repeatedly warning against the Tsai-McCarthy meeting.
"China's sovereignty and territorial integrity will never be divided," Foreign Ministry spokeswoman Mao Ning said at a regular press briefing.
"The future of Taiwan lies in reunification with the motherland."
AFP journalists on Pingtan island, China's nearest outpost to Taiwan, saw a military ship and at least three army helicopters transiting through the strait on Friday afternoon.
The silver-grey Dongtuo-859 tugboat cruised southwards, about 1 km from the shore, while the two aircraft flew northwards over the waters at fairly low altitude.
It was not immediately clear if the movements represented an enhancement of normal patrols that Beijing conducts in the region.
Last August, China deployed warships, missiles and fighter jets around Taiwan for its largest show of force in years, following a trip to the island by Ms. McCarthy's predecessor Nancy Pelosi.
Beijing's response to the Tsai-McCarthy meeting has so far been more muted and China's ADIZ incursions in the past two days have not risen beyond the activities of an average week.
The ADIZ is not the same as Taiwan's territorial airspace, including a far greater area that overlaps with part of China's own ADIZ and even some of the mainland.
Taiwan was on alert, with Premier Chen Chien-jen saying on Friday that its defence and security agencies were keeping a close eye on developments.
On Thursday, Taipei's Defence Ministry said three warships had been detected around the Taiwan Strait and one Chinese naval helicopter crossed the island's ADIZ.
The display prompted calls from the U.S. asking China "to cease its military, diplomatic and economic pressure against Taiwan and instead engage in meaningful diplomacy".
Taiwan also said Chinese coast guard vessels were "obstructing" trade by carrying out on-site inspections on cargo and passenger ships.
Ms. Tsai told reporters before she left the U.S. that her government hoped to do its "best to maintain peace and stability between the two sides".
"I also hope the Chinese side can exercise self-restraint and (not) overreact," she added.
Mr. McCarthy, who is second in line to the U.S. presidency, had originally planned to go to Taiwan himself but opted instead to meet Ms. Tsai in California.
The decision was viewed as a compromise that would underscore support for Taiwan but avoid inflaming tensions with China, a move analysts say has so far proven successful.
Mr. McCarthy had vowed that U.S. arms sales to Taiwan — which have infuriated the Chinese leadership — would continue, in what he said was a proven strategy to dissuade aggression.
"It is a critical lesson that we learned through Ukraine, that the idea of just sanctions in the future is not going to stop somebody" who wants to wage war, he said.
On Friday, China slapped sanctions on Taipei's de facto ambassador to the U.S., Hsiao Bi-khim, banning her from entering China and accusing her of "deliberately inciting cross-strait confrontation".
Taiwan condemned the move, saying Beijing was attempting to "further suppress our country's international space".
Beijing also announced sanctions against the Hudson Institute, a Washington-based conservative think tank, as well as the Ronald Reagan Presidential Library, which hosted the meeting.
The two organisations are now barred from engaging in transactions and cooperation with Chinese entities.
