A creature described as a "sea animal" chewed off both arms of a Russian swimmer in an extremely rare attack in the waters off its Far Eastern coast.

"A 25-year-old man was brought in in grave condition, unconscious. His arms were chewed off at the elbows," the Interfax news agency quoted a source in a hospital in the Khasan district of the Far Eastern Primorye region as saying.

"He'll live," the source added.

A local law enforcement source told Interfax that a shark attacked the man 50m off the shore.

"A witness helped the victim to get out of the water and also called rescue workers and medics," it said.

A spokesman for the local branch of the emergencies ministry confirmed to AFP that a "sea animal" attacked a man in the southern Khasan region but declined to provide further details.

Another local spokesman said officials expected to receive more details of the attack after hoping to speak to the man today.

Following the attack, local emergencies ministry officials toured the area warning thousands of swimmers to watch out for sharks, Interfax said.

Several types of sharks, including the herring shark, can be spotted in the northwestern part of the Sea of Japan but they do not attack people and swimming there is considered generally safe.

The emergencies ministry spokesman told AFP his ministry has never before registered a shark attack on a human in the Primorye region.

Meanwhile a woman vacationing in Puerto Rico was mauled by a shark in the bioluminescent bay of Vieques, requiring emergency surgery for wounds to the leg, officials said.

Lydia Strunk, 27, was bitten by a shark in waters off the Puerto Rican island of Vieques and airlifted to San Juan's Puerto Rico Medical Center, said emergency services official Teudy Martinez.

The bite was around 30cm long from the ankle to knee near the back of her right leg and damaged four tendons of the right foot, but hospital officials said she would be able to walk.

Strunk, a former vice president of Amnesty International's US chapter, will need to remain hospitalised for four to five days, doctors said.

Earlier this week, a British diver was killed by shark while snorkelling on his honeymoon in the Seychelles.

That was the second fatal shark attack in the region in just over a fortnight.