BEIJING (Reuters) - A formation of Chinese warships has been holding daily combat drills for more than a week in waters near Taiwan, China’s state media said on Tuesday, amid heightened tension between Beijing and the self-ruled island.

The news comes as U.S. Defense Secretary Jim Mattis was set to arrive in the Chinese capital for an official visit.

Since June 17, a group of navy warships, including a Type 054A frigate and a Type 052C destroyer, have been conducting exercises near Taiwan, including in the Bashi Channel and the Taiwan Strait, said 81.cn, an official publication of the Chinese army.

“The drills tested the military and training abilities of warship, aviation and coastal defense troops, via organizing real combat training in multiple areas of the ocean,” it said.

It was not clear if the drills had ended.

Taiwan’s defense ministry said in a statement the vessels were monitored continuously and there was no cause for alarm.

China claims Taiwan as its own and has never renounced the use of force to bring under its control what it sees as a wayward province. Taiwan has shown no interest in being governed by the ruling Communist Party in Beijing.

United States overtures towards Taiwan, from unveiling a new de facto embassy to passing the Taiwan Travel Act, which encourages U.S. officials to visit, have further escalated tension between Beijing and Taipei.

Sino-U.S. ties are under growing pressure over burgeoning trade friction, the North Korean nuclear crisis and escalating activity in the disputed waterway of the South China Sea.

Early in June, Mattis, in a strongly worded speech at the annual Shangri-La Dialogue in Singapore, warned of Chinese intimidation in the South China Sea, adding that the United States was ready to “compete vigorously” if needed.

China often says the United States’ acknowledgement of its “one-China” policy is foundational for two-way ties, and that Taiwan is the most sensitive issue in the relationship.

The United States is considering sending a warship through the Taiwan Strait, U.S. officials said in early June. Such a passage, should it happen, could be seen in Taiwan as a fresh sign of support by President Donald Trump.

The last time a U.S. aircraft carrier transited the Taiwan Strait was in 2007, during the administration of George W. Bush, and some U.S. military officials believe a carrier transit is overdue.

In recent months, China’s air force has held military maneuvers near the island, which Taipei has denounced as intimidation.

China’s hostility toward Taiwan has grown since Tsai Ing-wen from the pro-independence Democratic Progressive Party won presidential elections on the island in 2016.