A United Nations nuclear watchdog report says Iran continues with its uranium enrichment drive, a move certain to further fuel Western alarm about Tehran's atomic ambitions.
The International Atomic Energy Agency (IAEA) said on Friday it has "major differences" with Iran and "major concerns" about its nuclear programme after inspectors probing suspected weapons work returned from Tehran empty-handed.
In a confidential document, the atomic agency also reported that its mission to Tehran this week, to try and get Iran to respond to allegations of conducting research for the development of nuclear weapons, was in vain.
The IAEA report to member states showed Iran had carried out a significant expansion of activities at its main enrichment plant near the central city of Natanz and also increased work at its Fordow underground facility.
Enriched uranium can be used to fuel nuclear power plants, which is Iran's stated aim, or provide material for bombs if refined much further, which the West suspects is Tehran's ultimate aim.
Vladimir Putin, the Russian prime minister, responded to the report by accusing the West of attempting to institute "regime change" in Iran.
"Under the guise of trying to prevent the spread of weapons of mass destruction, they are attempting something else entirely and setting different goals: regime change," news agencies quoted Putin as saying.
"We have such suspicions, and we are trying to take a stand that differs from the one they are trying to force on us... concerning the ways that the Iranian nuclear problem might develop," Putin reportedly said.
Russia has longstanding commercial and military ties with Iran and has condemned recent sanctions imposed by the United States and the European Union over its suspected pursuit of nuclear arms.
The US and its chief regional ally Israel have never ruled out a strike against Iran over its suspected nuclear weapons development. Russia has always insisted the standoff can only be solved through diplomacy.
Iran is the third-largest holder of the world's oil reserves but has come under renewed sanctions from the US and the EU because of the strained relations.
More than one-third of the US senate has sponsored a non-binding resolution that lists all the reasons Iran is a threat and urges President Barack Obama to reaffirm that the US position is to “prevent the Iranian government from acquiring nuclear weapons capability.
Al Jazeera's Patty Culhane, reporting Washington, said: "This resolution says capability, not intent. And that is an important distinction because the administration says there is no evidence that Iran has decided it wants to build a bomb."
She continued: "This resolution says what matters is not what Iran wants to do, it's what it can do. And this latest report from the IAEA says Iran has some of the capabilities needed to build a bomb."