The vessel's nuclear plant was not damaged in the accident which happened while the San Francisco was conducting underwater operations 560 kilometers (350 miles) south of its base at Guam, the Navy said.

The submarine was heading back to base under its own steam but on the surface on Saturday.

The Los Angeles class submarine, which has a crew of 137, was heading for a port visit in Brisbane, Australia when the accident happened on Friday.

A Pacific fleet spokeswoman said about 20 crew were injured, and one was reported in critical condition.

The injured were being treated on board by medics with special emergency training, the spokeswoman said.

But the submarine was still too far out of helicopter range for the evacuation of the most seriously injured sailor, Pacific Fleet spokesman Master Chief John Barnett said.

"We are sending air and sea units out to meet the ship to bring in extra medical help," he pointed out. "We are still working trying to get the right assets out there to help those guys."

A Navy statement said the submarine was on the surface and making best speed back to their homeport in Guam.

"There were no reports of damage to the reactor plant, which is operating normally," according to the document.

Officials said the hull of the vessel was intact.

The submarine was expected to arrive back in port on Monday, but a full investigation into the accident has already started, officials said.

Los Angeles class submarines are 110 meters (360 feet) long and have one nuclear reactor and one shaft, according to US Navy data.

Submarines of that class are able to get on station quickly, stay for an extended period of time and carry out a variety of missions including deployment of special forces, minelaying, and precision land attacks.

The USS San Francisco is one of three submarines of its class to be based in Guam. It has been there since 2002. It can carry out intelligence gathering and take special forces on missions. Its strike arms usually include Tomahawk missiles.

The incident occurred at 0200 GMT Saturday (12 noon in Guam), the statement said.

The US Navy insists its submarines have an excellent safety record.

In 2001, the USS Greeneville collided with a Japanese fishing boat off Hawaii, killing nine Japanese boys and men. The commander was later reprimanded and had to retire.

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