Taiwan’s coastguard has detained a cargo ship and its Chinese crew after an undersea cable in the Taiwan Strait was damaged on Tuesday, saying it cannot rule out the possibility it was a deliberate “grey zone” act.

“Whether the cause of the undersea cable breakage was intentional sabotage or a simple accident remains to be clarified by further investigation,” the coastguard said in a statement. “It cannot be ruled out that it was a grey-zone intrusion by China.”

“Grey zone” refers to an act of hostile interference which does not reach the threshold of warfare.

The coastguard said it was alerted in the early hours of Tuesday by Taiwan’s Chunghwa Telecom company that a communications cable connecting Taiwan’s main island with Penghu, in the Taiwan Strait, had been damaged.

A coastguard spokesperson told the Guardian that areas near undersea cables had been under close watch by the Taiwanese authorities. He said that on 22 February the cargo ship was detected stopping “very close” to the Penghu cable, off the coast of the city of Tainan. Coastguard vessels were sent to the site at about 2.30am on Tuesday, which noticed that the cargo ship had dropped anchor. It started to move north-west about half an hour before Chunghwa Telecom reported the cable had been cut, the spokesperson said.

The vessel did not respond to attempts by the authorities to communicate with it by speaker and radio, the coastguard told the Guardian. It was then intercepted and escorted back to Anping port shortly after midday. Footage provided by the coastguard shows several officers boarding the cargo ship.

The ship is registered under a Togolese flag, but Taiwan’s coastguard said that was probably a flag of convenience and all eight crew onboard were Chinese nationals. Ownership of the cargo ship is unclear – tracking and registration data shows at least three different ship names associated with the vessel’s maritime identification number. The coastguard said the vessel was “China-funded” but did not elaborate.

Taiwan has drawn up a list of more than 50 ships for close monitoring, which it believes are sailing under flags of convenience, including from Togo, but are owned by Chinese companies. The list, first reported by the Financial Times and since confirmed by the Guardian, includes boats that have lingered in or near Taiwan’s territorial waters for significant amounts of time.

Chunghwa said a back-up cable had come online and communication was not affected.

It is the latest undersea cable to be damaged around Taiwan in recent years. In January, the Taiwanese authorities investigated a Chinese-owned, Cameroon-registered vessel, Shunxing 39, which was suspected of dragging its anchor and damaging a cable north-east of the island that ran to the US.

Taiwan’s coastguard had ordered the vessel to return to Taiwanese waters for investigation but was unable to board due to rough weather. The Shunxing 39 then sailed to South Korea.

In February 2023, damage to two cables near the outlying Matsu islands, close to the Chinese mainland, left residents without internet access for weeks. Two Chinese ships were blamed for cutting the cables in two incidents almost a week apart. However, the government stopped short of calling it a deliberate act on behalf of Beijing.