Iran has responded defiantly to new sanctions imposed by the US targeting Iran's Islamic Revolution Guards Corps and three state-owned banks.
The Iranian foreign ministry said the sanctions were doomed to failure.
US Secretary of State Condoleezza Rice says the measures are to "confront the threatening behaviour of the Iranians".
But both China and Russia criticised the sanctions. Russian President Vladimir Putin likened the US move to "mad people wielding razor blades".
Earlier, US Assistant Secretary of State Nicholas Burns criticised Russia for selling weapons to Iran and China for investing in the country.
He told the BBC: "It's very difficult for countries to say we're striking out on our own when they've got their own policies on the military side, aiding and abetting the Iranian government in strengthening its own military."
'Doomed to failure'
Iran's foreign ministry condemned the sanctions.
Officially the Islamic Revolution Guards Corps (IRGC), or Pasdaran
Formed after 1979 revolution
Loyal to clerics and counter to regular military
Estimated 125,000 troops
Includes ground forces, navy, air force, intelligence and special forces
Also has political influence: dozens of ex-guard sit as MPs
Iran President Ahmadinejad is a former member
Spokesman Mohammad Ali Hosseini said: "The hostile American policies towards the respectable people of Iran and the country's legal institutions are contrary to international law, without value and, as in the past, doomed to failure."
The head of the Revolutionary Guards, Mohammad Ali Jaafari, said the corps was ready to defend the ideals of the revolution more than ever before.
Correspondents in Tehran say the sanctions could be very damaging for Iran as the Revolutionary Guards are thought to control a third of the country's economy and foreign firms may now be deterred from dealing with them.
Mr Putin's comment came ahead of an EU-Russia summit in Mafra, near the Portuguese capital Lisbon.
BBC Europe editor, Mark Mardell, says that behind the president's colourful language, diplomatic sources say there is a real Russian irritation, a belief that new sanctions are the wrong approach and only make Iran less likely to give up its nuclear programme.
Russia is helping Iran construct a nuclear reactor.
On Friday, China's foreign ministry said Beijing was "opposed to imposing sanctions too rashly in international relations", saying it "can only make the situation more complicated".
Western nations suspect Iran is trying to build nuclear weapons, although Tehran says its programme is purely peaceful.
Ms Rice tried to play down any rift with Russia, saying neither wanted a nuclear-armed Iran.
"After all, Moscow is a lot closer to Iran than the United States," she said.
She strongly defended the sanctions, saying: "The international community cannot just sit idly by... A nuclear weapon in the hands of the Iranian regime would be deeply destabilising in the world's most volatile region."
Mr Burns said that despite differences with both Russia and China the US still hoped that the UN Security Council would approve a third resolution imposing new sanctions this November.
The US has repeatedly accused Iran of destabilising Iraq and Afghanistan, blaming the Revolutionary Guards for supplying and training insurgents.
The BBC's Justin Webb in Washington says Condoleezza Rice continues to be committed to finding a diplomatic resolution to the crisis.
However, he says Vice-President Dick Cheney is widely believed to be pressing for a military strike on Iran before the Bush administration's term is over, and if these sanctions have no effect, Ms Rice may well have to give way to his strategy.