Two US Navy warships are sailing through international waters in the Taiwan Strait, three US officials told Reuters, the first such operation since heightened tension with China over US House Speaker Nancy Pelosi’s visit to Taiwan.

In recent years US warships, and on occasion those from allied nations such as Britain and Canada, have routinely sailed through the strait, drawing Beijing’s anger.

China, which claims Taiwan as its own territory against the objections of the democratically elected government in Taipei, launched military drills near the island after Pelosi visited in early August, and those exercises have continued.

The trip infuriated Beijing, which saw it as a US attempt to interfere in China’s internal affairs.

The officials, speaking on condition of anonymity, on Saturday said US Navy cruisers Chancellorsville and Antietam were carrying out the operation which was still underway.

Such operations usually take between eight and 12 hours to complete and are closely monitored by the Chinese military.

The narrow Taiwan Strait has been a frequent source of military tension since the defeated Republic of China government fled to Taiwan in 1949 after losing a civil war with the communists, who established the People’s Republic of China.

Pelosi was followed around a week later by a group of five other US lawmakers, with China’s military responding by carrying out more exercises near Taiwan.

Senator Marsha Blackburn, a US lawmaker on the Senate Commerce and Armed Services committees, arrived in Taiwan on Thursday on the third visit by a US dignitary this month, defying pressure from Beijing to halt the trips.

The Biden administration has sought to keep tension between Washington and Beijing, inflamed by the visits, from boiling over into conflict, reiterating that such congressional trips are routine.

The United States has no formal diplomatic relations with Taiwan but is bound by law to provide the island with the means to defend itself.

China has never ruled out using force to bring Taiwan under its control.

Taiwan’s government says the People’s Republic of China has never ruled the island and so has no right to claim it, and that only its 23 million people can decide their future.