The Togolese-registered ship Hongtai was "escorted" back to Taiwan and the case was being "handled in accordance with national security-level principles", the coast guard said in a statement.

Taiwan's Chunghwa Telecom reported the cable between Penghu, a strategic island group in the sensitive Taiwan Strait, and Taiwan was disconnected early Tuesday, the Ministry of Digital Affairs said.

"Whether the cause of the undersea cable breakage was intentional sabotage or a simple accident remains to be clarified by further investigation," the coast guard said.

The ship was using a flag of convenience, was crewed by eight Chinese nationals and had Chinese funding, it said.

"It cannot be ruled out that it was a grey zone intrusion by China," the coast guard said, referring to actions that fall short of an act of war.

"The Coast Guard will cooperate with the prosecutors in the investigation and make every effort to clarify the truth."

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