The dredger is just one of about 1,200 mainland vessels that have operated in Taiwanese-administered waters since January, according to the island’s coastguard.

China-US close encounters ‘raise conflict risk in South China Sea’

Military analysts said the strategy was also a warning to Washington, which has strengthened its support of the island despite official diplomatic recognition of Beijing.

Beijing considers Taiwan a wayward province that must be returned to the mainland fold by force if necessary. It has time and again warned Washington against supplying arms for and forging partnership with the island.

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China warns US of ‘strong response’ regarding Taiwan, saying island’s independence is 'dead end'

China warns US of ‘strong response’ regarding Taiwan, saying island’s independence is 'dead end'

The activities of the mainland vessels near Penghu Island have raised particular concern in Taipei because the area, known as the Taiwan Bank, is seen as a potential gateway for mainland forces to enter Taiwan proper.

Chen Po-wei, a legislator of the pro-independence Taiwan Statebuilding Party, said mainland vessels had operated illegally at the Taiwan Bank since 2014.

In a question to military officials at a legislative session last month, Chen said: “By continuing the dredging there, would that create any security problem like digging an area huge enough to accommodate an ambush of our forces by a PLA submarine in the future?”

The officials said it would be rather unlikely because it would take years to achieve but they could not rule out any political or military motive behind the activities.

Beijing sends warplane to buzz Taiwan after missile test

Alexander Huang Chieh-cheng, a professor of strategic studies and international relations at Tamkang University in Taipei, said the PLA’s fly-bys of Taiwan had a number of benefits for the mainland military.

“First, they create a ‘pattern’ and a ‘new normal’ if not a ‘fait accompli’ that ‘disregards’ the unspoken understanding between the two sides for decades,” Huang said, referring to the understanding that the two sides avoid crossing a median line in the Taiwan Strait.

“Second, it can be seen as ... psychological operations targeting Taiwan’s general public [to create panic].”

He said that in the process, the PLA was also exhausting Taiwan’s air defence personnel, and testing and collecting signal data from the island’s defence systems.

“[Lastly] it helps the PLA to collect and better understand the hydrology between Taiwan and the Philippines, and between the South China Sea and the Taiwan Strait,” Huang said.

‘Grey zone’ tactics are raising risk of military conflict in the South China Sea, observers say

William Chung, a researcher of the government-funded Institute for National Defence and Security Research, said Beijing was expected to make even greater use of such tactics in the future.

Chung said a similar strategy was at play in the announcement of a 2½-month military drill in the Bohai Gulf from May 14 and a reported plan to conduct an amphibious landing drill near the Taiwan-controlled Dongsha Islands – also known as the Pratas – in August.

Chung’s colleagues, Paul Huang and Hung Ming-te, said the mainland was also using armed fishing vessels to defend its interests while preventing the targeted party from responding militarily.

In a report released on June 5, Paul Huang and Hung said the mainland’s maritime militia had been harassing or attacking vessels from other nations in the region, and Taiwan must take steps to counter the threat.

Recent examples included a dozen mainland fishing boats ramming a Taiwanese coastguard vessel in a coordinated ambush in waters near Quemoy on March 16, a Chinese coastguard vessel sinking a Vietnamese fishing boat on April 2, and several Chinese fishing boats ramming a Japanese destroyer on March 30.