However, it was unclear whether there was enough oxygen aboard the mini-sub to keep the crew alive long enough for the remote-controlled U.S. vehicle to reach them from its base in San Diego.
The Russian sub's propeller became entangled in a fishing net Thursday, Russian navy Capt. Igor Dygalo said on state-run Rossiya television. The accident occurred in Beryozovaya Bay, about 50 miles south of Kamchatka's capital, Petropavlovsk-Kamchatsky, officials said.
"There is air remaining on the underwater apparatus for a day — one day," he said at about 6 a.m. EDT. "The operation continues. We have a day, and intensive, active measures will be taken to rescue the AS-28 vessel and the people aboard."
Russian Pacific Fleet spokesman Capt. Alexander Kosolapov said contact had been made with the sailors, who were not hurt.
Dygalo told NTV television that Russian authorities were working out a rescue plan that could be put in motion later in the day, but he did not describe it.
He also said a British plane carrying unspecified rescue equipment was being sent but was not expected before Saturday.
The mini-sub, called an AS-28, was too deep to allow the sailors to swim to the surface on their own or for divers to reach it, officials said.
The accident occurred early Thursday after the mini-submarine was launched from a rescue ship during a combat training exercise, Kosolapov said. The AS-28, built in 1989, is about 44 feet long and 19 feet high and can dive to depths of 1,640 feet.
Two surface ships were sweeping the area with nets in the hope of wresting the trapped vessel from the sea floor, adding that the rescue effort would continue into the night, Dygalo said.
Russia appealed to the United States and Japan for assistance, the Interfax news agency quoted Foreign Ministry spokesman Boris Malakhov as saying.
At least one robotic rescue vehicle from San Diego will be shipped on a plane Friday to Russia to help save the submarine. The unmanned vehicle, called a Super Scorpio, can reach depths of up to 5,000 feet and is equipped with high-powered lights, sonar and video cameras, said Capt. Matt Brown, a spokesman for the U.S. Navy's Pacific Fleet in Honolulu.
The Deep Submergence Unit team is scheduled to leave San Diego's North Island Naval Air Station on an Air Force C-5 transport plane at 1:45 p.m. EDT, the Pentagon said.
The Super Scorpio then will be transported by truck and loaded on a Russian ship before making its descent to the stricken vessel.
Brown said the Russian military has indicated that the AS-28 may have been fouled by fishing nets or steel cables. The vehicle does have an instrument that can cut steel cables, he said.
The Super Scorpio, which weighs about 4,500 pounds, has been used to conduct underwater surveys and inspections.
About 30 people will accompany the vehicle to Russia, Brown said.
"We are working as fast as we can to make this happen," he said.
Since Soviet times, the Kamchatka Peninsula has housed several major submarine bases and numerous other military facilities, and large areas of it have remained closed to outsiders.
Airlifting a U.S. underwater vehicle to the area will mark the first time since the World War II era that a U.S. military plane has been allowed to fly there.
At Moscow's request, Japan dispatched a vessel carrying submarine rescue gear and three other ships to join salvage efforts, but they weren't expected to arrive at the scene until early next week, Marine Self Defense Force spokesman Hidetsubu Iwamasa said.
The accident occurred almost exactly five years after the nuclear submarine Kursk sank to the bottom of the Barents Sea after explosions on board, killing all 118 seamen aboard. Some Kursk sailors survived for hours after the accident as oxygen ran out, and Russian authorities came under sharp criticism for their handling of the crisis.
The same type of vessel that is now stuck, called a Priz, was used in the rescue efforts that followed the Kursk disaster, Interfax reported.