WASHINGTON (Reuters) - A senior Russian naval officer has discounted a theory that a U.S. vessel hit the Kursk and said he believed an onboard explosion had ripped through the sunken nuclear submarine.

Russian vice admiral Sergei Lebedev told reporters he thought the vessel sank to the bottom on the Barents Sea because of an explosion last August in the front section of the submarine, which killed all 118 sailors on board.

Asked to comment on a theory that the Kursk had collided with a U.S. vessel in the region, he said this was "highly unlikely." He also ruled out that the vessel had hit a mine in the seabed of the Barents Sea.

"I believe it is very unlikely that the Kursk struck another vessel ... because of the nature of the explosion it came from within," he told reporters at a briefing at the Russian Embassy to discuss the salvage operation of the Kursk. Lebedev stressed his views were personal and that the exact reason would be known only when the findings of an official investigation were made public.

Russia shows underwater footage of work on sunken Kursk submarine

MOSCOW (AP) - Divers using special drills and chains will start slicing off the mangled front section of the sunken nuclear submarine Kursk on Aug. 12 - the first anniversary of the disaster that destroyed the huge vessel, officials said.

The Russian Navy released video footage of the underwater preparations to lift the Kursk from the Barents Sea floor next month. The video, shot Aug. 2 and shown on Russian television stations Thursday, showed divers' hands maneuvering outside the hull and amid a mass of wires and pipes between the outer and inner hulls.

It also shows the process of opening and removing cut-out pieces of hull, and cables and onboard systems in the fifth compartment, Navy spokesman Igor Dygalo said.

The fifth compartment contains the Kursk's nuclear reactors. Russian officials insist there is no risk that the reactors will be damaged in the risky lifting operation, scheduled for mid-September.

Before the submarine is lifted, divers will sever the front section, which Russian officials say may contain unexploded torpedoes. That section will be left on the sea floor, though Russian officials say they may raise it later.

Dygalo said divers will start the cutting Aug. 12, the ITAR-Tass news agency reported.

The Kursk suffered explosions during exercises Aug. 12, 2000, and plunged to the sea bottom. All 118 men aboard were killed. A memorial service is to be held Sunday on the anniversary at the Kursk's home base in the port of Vidyayevo.

Divers continued cutting holes Thursday in the Kursk's double hull, to which steel cables will be attached to raise the submarine. The cables will be connected to hydraulic lifting devices anchored to a giant barge, which will bring the Kursk to the Arctic port of Murmansk.

The cause of the explosions remains unclear. Russian officials say it could have been prompted by a collision with a World War II mine or a Western submarine, though outside experts believe it was likely an internal malfunction.