The event came amid growing hostility between China and Vietnam, with some experts warning the submarine could have surfaced as a warning. According to international submarine warfare analyst H.I. Sutton, the Chinese 11,000-ton Type 094 Jin Class submarine suddenly emerged from the deep among a fishing fleet operating off the Paracel Islands. The Paracel Islands have been at the centre of disputes between Beijing and Hanoi – with both claiming ownership of the cluster. Mr Sutton highlighted that submarines of this type are designed and used to remain underwater for months on end.
He added that they are also used by Beijing to deter any surprise attack.
Mr Sutton argued that using such a submarine to “send a message” to Vietnam goes against all expectations.
He said: “Surfacing next to another country’s vessel is unusual and suggests that something has gone wrong.
“Something serious enough to warrant sacrificing its main asset: stealth.”
China and Vietnam have a long history of disputes and the troubled relationship is pock-marked with violence. Current disagreements focus on the oil and mineral rich waters and islands that make up the South China Sea.
Last year, a Chinese oil survey vessel – The Haiyang Dizhi 8 – became embroiled in a tense standoff with Vietnamese vessels in the, but left Hanoi-controlled waters after more than three months, marine data showed.
The Vietnamese also fought China in a deadly war in 1979 and, in 1988, a skirmish saw 64 Vietnam soldiers killed by Beijing’s People’s Liberation Army.
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Photographs published by the Philippine Daily Inquirer showed cargo ships and supply vessels which appeared to be delivering construction materials to the China-controlled islands.
Others show runways, hangars, control towers, helipads and radomes as well as a series of multistorey buildings that China has built on reefs.
Fears remain focussed at China’s recent aggression with its weaponry in the region sparking fears of conflict.
A source close to the Chinese military said last month that an “aircraft-carrier killer” and one other missile were launched into the South China Sea as a warning to the US.
One of the missiles, a DF-26B, was launched from the northwestern province of Qinghai, while the other, a DF-21D, lifted off from Zhejiang, a province in the east of the country.
The move represents a drastic escalation in an already fragile standoff between two of the world’s biggest nuclear powers.