BEIJING, Aug 28 (Reuters) - China's military said on Sunday it was monitoring U.S. Navy vessels sailing through the Taiwan Strait, maintaining a high alert and ready to defeat any provocations.

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

The U.S. Navy said the guided-missile cruisers USS Antietam and USS Chancellorsville conducted a "routine Taiwan Strait transit" on Sunday, in accordance with international law.

In recent years U.S. 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 U.S. attempt to interfere in China's internal affairs.

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.