Taiwan leader Tsai Ing-wen held talks with McCarthy in Los Angeles on Wednesday, expressing gratitude afterwards for the meeting that included other US lawmakers.

"Their presence and unwavering support reassure the people of Taiwan that we are not isolated and we are not alone," Tsai told reporters.

China had repeatedly warned both sides the meeting should not take place, and deployed an aircraft carrier through waters southeast of Taiwan hours before the talks.

Three additional warships were detected in waters separating the island from mainland China, Taiwan's Ministry of National Defence said on Thursday morning.

An anti-submarine helicopter had also crossed the island's air defence identification zone, according to the ministry.

And China deployed coastguard vessels for unusual patrols that sparked a protest from Taiwan.

Despite Taiwan having been ruled separately for more than 70 years, China views it as part of its territory and has vowed to one day seize it, by force if necessary.

China carried out its largest-ever air and sea exercises around Taiwan following a visit in August last year by McCarthy's predecessor, Nancy Pelosi, to the island.

China then deployed warships, missiles and fighter jets into the waters and skies around Taiwan.

Its response to the McCarthy meeting has so far been on a much lower level, but still left Taiwan on high alert.

Taiwan's defence minister described the timing of the deployment of the Shandong, one of just two Chinese aircraft carriers, as "sensitive".

"When an aircraft carrier comes out, there are usually takeoffs and landings for aircraft but we have not found any takeoff or landing. We will keep watching," Chiu Kuo-cheng told reporters.

When asked if Shandong's deployment was a prelude to Chinese military exercises, Chiu said: "We are not ruling this out".

Chinese President Xi Jinping is currently hosting French counterpart Emmanuel Macron and European Commission chief Ursula von der Leyen as the EU attempts to repair relations with Beijing.

- 'Unwavering support' -

Tsai's visit to California was technically a stop-over after a trip to Latin America to see two of Taiwan's dwindling band of official diplomatic allies.

China had repeatedly issued warnings ahead of the Tsai-McCarthy meeting and issued another strong rebuke Thursday afternoon.

"China will take firm and forceful measures to firmly safeguard national sovereignty and territorial integrity," foreign ministry spokeswoman Mao Ning said at a regular briefing.

McCarthy, who is second in line to the US presidency, had originally planned to go to Taiwan himself but opted instead to meet Tsai in California.

The decision was viewed as a compromise that would underscore support for Taiwan but avoid inflaming tensions with China.

McCarthy said after meeting Tsai that a shared belief in freedom and democracy underpinned a relationship that was "a matter of profound importance to the free world".

McCarthy vowed US arms sales to Taiwan -- which infuriate Chinese leadership -- would continue, in what he said was a proven strategy to dissuade aggression.

"And what we know through history, the best way to do that is supply the weapons that allow people to deter war," he said.

"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.

An eight-member US congressional delegation also arrived in Taiwan on Thursday to hold talks on trade and security -- such visits have increased in recent years.

- China pressure -

There were no initial signs of extra military activity on Thursday morning on Pingtan island in southeastern China -- home to a People's Liberation Army base and known as the closest point on the mainland to Taiwan.

AFP journalists on Pingtan last year had witnessed missile launches and army helicopters flying over the island following Pelosi's visit.

However, Taipei's top China policy-making body, the Mainland Affairs Council, said Chinese coast guard vessels were "obstructing" trade by carrying out on-site inspections on cargo and passenger ships.

Chinese maritime authorities on Wednesday announced coastguard patrols in the Taiwan Strait.

"The Chinese side's action deliberately escalates cross-strait tensions," the Mainland Affairs Council said late Wednesday.

"(This) is a clear violation of the cross-strait shipping agreement and maritime practice which will have a serious adverse impact on the normal traffic between the two sides."

Taiwanese vessels have been ordered to refuse Chinese inspection demands, Taiwan National Security Bureau deputy director general Ko Cheng-heng said on Thursday.