The Russian diplomat arrived in Iran on a surprise visit, Western media said. The Russian Foreign Ministry has neither confirmed nor denied the reports.

Ryabkov, who is reportedly now in Tehran, is due to meet with Jalili, who is also secretary of Iran's Supreme National Security, later in the day.

According to Iranian media reports, the two officials will discuss the situation in Syria and a time frame for the next round of Iran’s negotiations with the group of international mediators.

Ryabkov has already met with Ali Bagheri, deputy secretary of Iran's Supreme National Security, IRNA said.

Ryabkov was quoted by the Financial Times on Sunday as saying Moscow would support bilateral talks between the United States and Iran on Tehran's nuclear program.

Last Friday, Iranian President Mahmoud Ahmadinejad reiterated his call for direct talks with the U.S., Iranian media reported.

The United States, Israel and some of their allies accuse Iran of pursuing non-civilian objectives in its nuclear energy program. Iran rejects the allegations, arguing that it is entitled to develop and acquire nuclear technology for peaceful purposes.

Russia has said it will scuttle any UN Security Council resolution that could be interpreted as allowing military action against Iran.