Naval engineers using a remote-control, deep-sea vessel conducted exploratory work Tuesday at the site where the Kursk nuclear submarine sank, laying the groundwork for a two-month operation to raise the shattered ship.

Seven ships including the Mayo, a Norwegian dive support ship, were at the site in the Barents Sea, about 93 miles away from the Russian Arctic port of Murmansk. They were joined Tuesday by an eighth ship, the Klavdiya Yelanskaya, carrying scores of journalists.

The exploration vessel from the Mayo began radiation checks Sunday, taking samples from the water and sea bed, to make sure the area is safe for divers to begin the operation to raise the Kursk.

The operation's commander, Vice Adm. Mikhail Motsak, said Tuesday it was "imperative to raise the Kursk. We must find all the submariners that died and give them a respectable burial."

The Kursk exploded and sank on Aug. 12, 2000, during a training exercise in the Arctic waters off northern Russia, killing all 118 crewmen aboard.

The operation to raise the submarine, which has two nuclear reactors and is believed to have unexploded torpedoes aboard, is scheduled to last through mid-September.

Russia has maintained that no radiation has leaked from the wreck and says it is raising it to ensure it poses no future danger. But nuclear safety officials in nearby Norway have said the operation's tight schedule increases the risk of a nuclear accident in the Arctic.

In Moscow, Russian Navy spokesman Igor Dygalo said that regular monitoring over the past year had shown no increased levels of radioactivity.

"We have seriously addressed the ecological aspect in the technical project of the operation," he said in an interview on NTV television.

Dygalo said that the unmanned vessel had mapped out the site where the Kursk lies.

"The maps show in detail the situation 50 meters (100 feet) around the submarine," he said. "Naturally, water and soil samples have been taken to check for radioactivity."

Meanwhile, Russian naval aviation chief Ivan Fedin said Tuesday that his pilots had spotted several foreign "underwater objects" trying to approach the Kursk and pointed them out to Russian Navy ships, which drove them out, the Interfax news agency reported.

Fedin wouldn't identify the objects or their countries of origin.

Russian officials hope raising the Kursk will enable them to learn more about the cause of the explosions and recover the remains of more of the crewmen. Only 12 were recovered during a salvage operation last fall.

"The (Russian) Northern Fleet has and will always think it is its sacred duty to determine the reason for the Kursk's sinking," said Motsak, the operation's commander.

"It is entirely possible that we can establish the accident's cause only when we lift the front section, which is planned for next year and will be done by the Russian side," he said.