Tehran, Jan 18 (Agencies): Iranian President Mahmoud Ahmadinejad and his Russian counterpart Dmitry Medvedev agreed to boost ties between their countries in a telephone conversation, state media reported on Monday.

The call came after Russia criticized Iran's invitation to visit two atomic sites in the Islamic republic that were thrown open to a group of foreign diplomats at the weekend.

It was also ahead of key talks between Iran and the world powers in Istanbul on Friday and Saturday.

Iran and Russia must always be "friends and colleagues," state television quoted Ahmadinejad as telling Medvedev. "The Islamic republic of Iran welcomes an increase of ties in all areas between the two countries."

Its website said Medvedev echoed the Iranian hardliner's views. He said the two long-standing allies could boost ties in energy, fuel, transportation and other commercial areas.

"Russia is ready for all cooperation with the Islamic Republic of Iran," Medvedev was quoted as saying.

On Saturday and Sunday, Iran opened up two atomic installations to a group of foreign envoys, including some representatives of the UN atomic watchdog.

It had invited Russia and China to participate in the visit to its main uranium enrichment facility at Natanz and the heavy water facility in Arak. But both Russia and China, considered close to Iran, turned down the invitation.

Russia and China backed a fourth set of UN sanctions against Iran approved in June 2009. The sanctions were imposed to stop Iran's nuclear programme, which the West suspects is masking a weapons drive, a charge Tehran denies.

Moscow will take part in this week's talks on the Iranian nuclear programme to be held in Istanbul.

Besides Russia, officials from the United States, Britain, France, China and Germany are to sit down with Iranian officials to ascertain whether the programme has peaceful aims as stated by Tehran.

Russia has been Iran's nuclear partner for years, but since last year is under pressure from the United States when it comes to the Islamic republic's atomic programme.

Moscow helped to build Iran's first civilian nuclear power plant in the southern port city of Bushehr.