Scientists suggested that a man-eater shark may be behind the extremely unusual attacks, and the authorities have banned swimmming in the far eastern Primorye region known for its sandy beaches and clear water.

A 16-year-old boy survived the latest attack but received serious injuries, including a tear wound to his thigh and damaged arteries, a local emergencies ministry spokesman told AFP, adding he was rushed for treatment to the regional capital Vladivostok.

The attack happened in the Khasan district where local authorities had earlier banned swimming after a shark on Wednesday mauled a 25-year-old man, chewing off his arms to the elbows in an attack that stunned locals.

“He was swimming with his wife and was attacked by a shark,” Arkady Babenko, head of surgery at the Khasan Central District Hospital, said of the 25-year-old who was bathing just 20-25 metres (yards) off shore in the Telyakovsky bay.

“He fought back as hard as he could, protecting his wife,” he said in televised remarks, adding that the patient was now in a grave condition in intensive care.

A shark attacked the 16-year-old youth off the nearby Zheltukhin island, also in the Khasan district, which shares a border with North Korea.

Scientists said shark attacks on humans were unprecedented in the region and the local emergencies ministry appeared so stunned after the first attack on Wednesday that it initially refused to confirm a shark attack, saying the man was attacked by a “sea animal.”

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.

While dangerous sharks including the white shark, also known as the man-eater shark, have been sighted in the region’s waters before, scientists and officials said they were unaware of shark attacks on humans.

“There have not been any instances of attacks on humans,” said Vladimir Rakov, a professor of marine biology in Vladivostok.

Citing witnesses who described Wednesday’s shark as being up to 4 metres (13-feet) long and having a barrel-shaped body, Rakov said that a white shark was likely behind the attack.

Scientists were hard-pressed to explain the attacks, saying the sharks might be drawn to the area by the warm clear waters and the populations of seals living on the islands.

Emergencies ministry officials have been touring the area — a favourite summertime playground — warning local holiday-makers against swimming.

“Following the decision of the head of the administration of the Khasan district, swimming is temporarily banned until the reasons for what happened are established,” the emergencies ministry said in a statement.

A local official, speaking on condition of anonymity, told AFP that the authorities would try to catch the shark, declining further details.