TAIWAN officials and defence analysts are bracing for intensifying pressure on the "median line" that has for decades helped keep the peace in the Taiwan Strait as China begins inspecting merchant shipping.

China's Fujian maritime safety officials have launched an inspection operation in retaliation for President Tsai Ing-wen's meeting in California with US House Speaker Kevin McCarthy, leader of the Republican majority House of Representatives, which is hostile to the Democratic administration.

The maritime safety authority in the southeastern Chinese province said the operation included "on-site inspections" on cargo ships and barges in the north and centre of the Taiwan Strait "to ensure the safety of vessel navigation and ensure the safe and orderly operation of key projects on water."

Taiwan's transport ministry said it had lodged a strong protest with Beijing and the island's defence minister, Chiu Kuo-cheng, told lawmakers that any Chinese boarding of Taiwanese ships would be illegal.

"As long as they are ships hoisting our country's flag they are all a part of our territory," he said.

Taiwan's military will not allow China to "unilaterally" board Taiwanese ships, he said.

China claims Taiwan as its own and says the Taiwan Strait is its sovereign territory and while China has never officially recognised the median line that a US general devised in 1954 at the height of Cold War hostility, China's military has for years largely respected it.

China's Taiwan Affairs Office did not respond on to requests for comment on Taiwan's criticism of the new patrols, or on whether it would use force to board Taiwanese ships or whether it would talk to Taipei to address its concerns.

The ship carrying out the patrols is the 128-metre long, 6,600-ton Haixun 06, operated by the maritime safety administration, rather than heavily armed naval or coast guard vessels.