A senior Israeli figure close to the intelligence services says his country was linked to the interception of a mystery Russian cargo ship in the Arctic Sea last month.
Russia says its navy intervened to end a hijack, but the Israeli source said the piracy story was a cover.
Israeli media reports say prime minister Benjamin Netanyahu made a secret visit to Moscow on Monday.
The BBC reports the source said Israel told Moscow it knew the ship was secretly carrying a Russian air defence system for Iran.
Russian Foreign Minister Sergei Lavrov has dismissed speculation that S-300 missiles were on board the ship.
Israeli Prime Minister Benjamin Netanyahu, meanwhile, has denied making a secret visit to Moscow on Monday.
Mr Netanyahu's office says he was visiting an Israeli security installation when he vanished from public view for much of the day.
The Arctic Sea and its 15 Russian crew vanished in July after leaving Finland with an apparent cargo of timber destined for Algeria. It was found on 16 August off West Africa.
Moscow announced that the Russian navy had captured the hijackers and rescued the crew. Eight men were later charged with hijacking and piracy.
However, the Israeli source told the BBC that the piracy story was a cover and that Israel told Moscow it was giving officials time to stop the shipment before making the matter public.
On Tuesday, Mr Lavrov called media speculation that S-300 missiles were on board the Arctic Sea "groundless". He also promised a "transparent" investigation into the disappearance of the ship.