Iran said on Tuesday it has managed to overhaul one of the three Russian-made submarines it has in its fleet, despite Moscow rejecting requests for plans and parts.

"At first it was a very difficult task. But with perseverance and determination, our navy proved that it was a doable task," the commander of the Iranian navy, Admiral Habibollah Sayari, was quoted as saying by the state television website.

The three Kilo class diesel-electric submarines were bought from Russia two decades ago as centrepieces of a navy that relies on them and half a dozen frigates or destroyers, most of which were bought before Iran's 1979 Islamic revolution.

The sub that was overhauled, "Tareq," had various parts changed, including engine components and the diving system, and the ailerons, propellers, pumps and electronic units, with replacements all made in Iran, Sayari said.

The Russians, he added, had refused to supply plans of parts. "They insisted that the submarine's repair be carried out in their country's shipyard," he said.

Iran declined that offer because it wanted independence militarily -- and also because "if we had returned this submarine to its manufacturer, perhaps we would not have had Tareq returned," the admiral said.

Iran's relationship with Russia is largely based on military purchases.

However, disputes between the two nations have been occurring since 2006, when Moscow signed on to UN resolutions calling for Tehran to curb its disputed nuclear programme.

In 2010, Russia backed out of a commitment to sell an advanced anti-aircraft missile system to the Islamic republic.