Russia is disappointed with Iran's failure to react "constructively" to the proposals made by a group of international mediators on Tehran's nuclear program, the Russian foreign minister said Wednesday.

"We have agreed to continue working toward a positive reaction [from Iran] to these proposals," Sergei Lavrov said. He offered no indication as to the specific nature of Iran's reaction.

However, he warned that "it is impossible to wait forever" and that Russia's partners "are already talking about the need to discuss further steps at the UN Security Council."

Mediators from the Iran Six - Russia, the United States, China, Britain, France and Germany - are currently discussing the transition from talks to new sanctions in response to Iran's refusal to fold its controversial nuclear program.

The minister stressed that "Iran's nuclear program is, without a doubt, an important subject of Russian-U.S. dialogue."

His remarks come in the wake of Tehran's decision to postpone a visit to Russia by Iran's top security official and chief nuclear negotiator.

Iranian news agencies reported on Monday that Saeed Jalili was due to in Moscow for a three-day visit that would include meetings with President Dmitry Medvedev and other officials to discuss bilateral relations, regional and international issues.

Russia has a history of close ties to Iran, including in nuclear energy through the construction of the Bushehr power plant, but is seen as having moved closer to the U.S. position on imposing new sanctions.

Iran, which is already under three sets of United Nations sanctions for refusing to halt uranium enrichment, recently announced plans to build 10 new uranium enrichment facilities. Western powers suspect it of pursuing an atomic weapons program.

The Iran Six- have long been trying to persuade Tehran to halt uranium enrichment not only through sanctions but also in exchange for economic and diplomatic incentives.

Iran Six envoys last met in New York on January 16, but in the face of Chinese resistance to a tougher course of action decided against immediate sanctions in the hope of finding new political and diplomatic solutions to the problem.