TAIPEI (Reuters) - The United States sent two warships through the Taiwan Strait on Thursday in the first such operation this year, the Taiwan government said, as it increases the frequency of transits through the strategic waterway to the concern of China.

Shiyu, or Lion Islet, which is part of Kinmen county, one of Taiwan's offshore islands, is seen in front of China's Xiamen, on Lieyu island, Kinmen county, Taiwan August 20, 2018. REUTERS/Tyrone Siu

The passage by the U.S. ships will likely be viewed in Taiwan as a sign of support from U.S. President Donald Trump’s government amid growing friction between the self-ruled island and Beijing.

China considers Taiwan its own and has not ruled out the use of force to bring it under its control.

Taiwan’s defense ministry said in a statement late on Thursday the ships were moving in a northerly direction through the waterway that separates Taiwan from the Chinese mainland and that their voyage was in accordance with regulations.

It said Taiwan closely monitored the operation to “ensure the security of the seas and regional stability”.

The United States sent ships through the waterway three times last year.

Despite Chinese military technological advances that pose an increased threat to U.S. warships, the U.S. Navy has said it has not ruled out sending an aircraft carrier through the strait, something it has not done in more than 10 years.

In Beijing, Chinese Foreign Ministry spokeswoman Hua Chunying said China closely monitored the U.S. ships through the waters, and had “expressed concern” to the U.S. side, urging it to abide by the “one China” principle.

China has stepped up pressure on Taiwan since President Tsar Ing-wen, from the pro-independence ruling party, took office in 2016. It has regularly sent military aircraft and ships to circle the island on drills in the past few years.

Beijing sent several bombers and aircraft through the Bashi Channel, which separates Taiwan from the Philippines, earlier on Thursday, Taiwan’s defense ministry said in a separate statement.

A similar Chinese operation was conducted on Tuesday, the ministry said, and both were monitored closely.

Chinese President Xi Jinping said in early January China reserves the right to use force to bring Taiwan under its control. In response, Tsai vowed to defend the island’s democracy and called for international support to protect Taiwan’s way of life.

Trump recently signed into law the Asia Reassurance Initiative Act reaffirming the U.S. commitment to Taiwan, including arms sales.

The United States recognizes only “one China” and has no formal ties with Taiwan, but it is bound by law to help the island defend itself and is its main source of arms.