Taiwan's defense ministry said Monday that Chinese naval vessels have been increasingly active in the Taiwan Strait this year as reports noted that "irregular" patrols are now "routine."

Analysts suspect that the patrols are a reaction to the US Navy's activities in the closely-watched strait.

The US Navy has sent warships through the Taiwan Strait three times this year, angering Beijing. The most recent incident occurred late last month.

The US Navy and Air Force have also conducted regular freedom-of-navigation operations and bomber overflights in the South China Sea, moves that have drawn repeated criticism from China.

Taiwan’s defence ministry confirmed on Monday that naval vessels from the mainland have stepped up patrols in the western part of the Taiwan Strait this year in what analysts say is a reaction to the increased number of US warships sent into the waters to test Beijing.

The ministry was responding to reports in the Taipei-based China Times on Sunday that “irregular” patrols by People’s Liberation Army Navy warships in the western side of the strait this year had become “routine”.

The ministry said the island’s forces had “effectively monitored the situations and movements around the Taiwan Strait by means of its air and naval mechanisms to ensure national security and regional stability”.

Beijing, which considers Taiwan a wayward province awaiting reunification by force if necessary, has suspended official exchanges with Taipei since President Tsai Ing-wen took office on the island in 2016 and refused to accept the one-China principle, which Beijing insists as a political foundation for cross-strait relations.

To try to force Tsai to accept the principle, Beijing has stepped up its military posturing against Taipei with a series of exercises around the island, in addition to poaching five of its allies.

A Taiwanese military source told the South China Morning Post that the PLA used to hold its patrols in waters close to the mainland side, but in recent years had moved closer to the midline of the strait.

“Each time the US sent warships through the Taiwan Strait, the PLA has also dispatched its fleets to track the US’ movements,” the source said, adding that the Taiwanese military also shadowed the mainland vessels.

Since July 7, the US has sent three batches of warships past Taiwan in “freedom of navigation” exercises in the strait and the South China Sea, where Beijing is building on disputed islands.

The first, on July 7, included two destroyers; the second, on October 22, involved a frigate and destroyer. On Wednesday, soon after the island’s local elections, the US sent a guided missile destroyer and an oiler through the strait ahead of the meeting between Chinese President Xi Jinping and his US counterpart Donald Trump at the G20 summit in Argentina.

Beijing reacted strongly each time. After last week’s action, both the mainland defence and foreign ministries expressed concerns and repeated Beijing’s stand on “protecting China’s sovereignty” in relation to Taiwan.