By Ben Blanchard and Yimou Lee
TAIPEI (Reuters) - 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 civilian shipping across the waterway.
China's Fujian maritime safety administration launched a three-day special patrol and inspection operation on Wednesday, which is seen in Taiwan as retaliation for President Tsai Ing-wen's meeting in California with U.S. House Speaker Kevin McCarthy.
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 on Thursday 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 U.S. general devised in 1954 at the height of Cold War hostility, China's military had for years largely respected it.
But the traditional role played by the imaginary line was thrown into question last year in the days after the then-U.S. Speaker Nancy Pelosi visited Taiwan.
China, infuriated by the visit, sent its navy ships on exercises to both sides of the line but it did not seek to directly stop or board civilian ships.
China's Taiwan Affairs Office did not respond on Thursday 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.
It is also not clear precisely what action Taiwan will take in response to the operation that some analysts as a new tactic in a "grey zone" strategy of attempting to wear down Taiwan with offensive actions that stop short of full-blown conflict.
'NOT CIVILISED'
A spokesperson for Taiwan's office overseeing relations with Beijing, the Mainland Affairs Council, said China's inspection operation was "not civilised" behaviour but they had not yet received reports of harassment or requests to board ships.
As to how Taiwan would respond if such actions took place, the spokesperson, Chan Chih-hung, said: "If you interfere, we will hit back". He did not elaborate.
A senior Taiwan official familiar with security planning said Taiwan would not allow China to board ships in the Taiwan Strait and that Taiwan's coast guard and military would jointly respond if China made a move to do so.
The ship carrying out the patrols is the 128-metre long, 6600-ton Haixun 06, operated by the maritime safety administration, rather than heavily armed naval or coast guard vessels.
Commissioning the ship in July last year, Chinese officials said the Haixun 06 would deal help smooth shipping flows and deal with accidents, distinguishing it from coast guard and navy vessels that would deal with "security risks" in the area, the state-backed Global Times reported at the time.
Chinese state television broadcast live pictures of the Haixun 6 on patrol, including shaky footage of a Taiwanese coast guard ship shadowing it in the distance. The footage did not show any vessels being stopped.
Taiwan's Ocean Affairs Council said it had sent a coast guard ship to monitor the Chinese vessel and "prevent mainland China from endangering the freedom of navigation and safety of our citizens in the waters under our jurisdiction with improper behaviour".
While Taiwanese navy ships shadowed the Chinese navy ships during the exercises in August, they did not directly confront them - action they might have been forced to consider if the Chinese ships had entered Taiwan's contiguous zone, 24 nautical miles (44.4 km) from its coast, security officials said at the time.
Chieh Chung, a military researcher at the National Policy Foundation think tank, said the inspections were designed to show that China could effectively exercise jurisdiction across the median line.
"Not only is it a 'legal battle' against us but if the scope of these patrols and checks crosses to the east of the median line, it will further negate the tacit understanding of that line," Chieh said.
"It will attract attention without greatly increasing military tensions."