"We agree on and share a kind of disappointment, and there are reasons for this," Ryabkov said, speaking on Thursday in the Nixon Center in Washington.

"Firstly, we have not decided on the date and place of the next meeting. Secondly, we have been bogged down because Iran demands the Iran Six recognize its right to enrich uranium and declare its willingness to drop sanctions," he went on.

However, he stressed there was no alternative to negotiations with Iran.

"We told the Iran representatives that we should reinforce mutual trust before discussing concrete agreements. We proposed some practical ideas," Ryabkov added.

Two-day Istanbul talks with Iran on its controversial nuclear program ended on January 22 with no breakthrough in sight, and the EU's foreign policy chief, Catherine Ashton, said the six world powers were "disappointed" with the negotiations.

"Our doors remain open but Iran is not ready for this," she said.

World powers, led by the United States, suspect Iran of pursuing a secret nuclear weapons program, but the Islamic Republic insists it needs nuclear power solely for civilian energy generation.

The Iran Six, which comprises Russia, the United States, China, Britain, France and Germany, has been trying since 2003 to convince Iran to halt its uranium enrichment program.