The US, UK and Canada have announced new sanctions against Iran amid growing concern over its nuclear programme.
US Secretary of State Hillary Clinton spoke of a "significant ratcheting-up of pressure" on Iran.
The UK said earlier it was cutting all ties with Iranian banks, while Canada said it was banning exports for the petrochemical, oil and gas industries.
A UN report has given the strongest evidence yet that Iran is seeking nuclear weapons, but Iran denies this.
Tehran insists its nuclear programme is solely for civilian purposes.
The report, by the UN's nuclear watchdog (IAEA), said Iran had carried out tests "relevant to the development of a nuclear device".
But despite the report, Iran was not referred to the UN Security Council because Russia and China were opposed to the move.
Russia has described the new sanctions as "unacceptable and against international law".
"This practice seriously complicates moves for constructive dialogue with Tehran," said a statement from the Russian Foreign Ministry.
"We believe that the constant stepping up of sanctions has long since transcended the boundaries of solving non proliferation tasks in the context of the Iranian nuclear programme," the statement said.
Mrs Clinton said that Iran's petrochemical industry, oil and gas industry and financial sector would be targeted by the sanctions.
"The message is clear," she said. "If Iran's intransigence continues, it will face increasing pressure and isolation. Today, the United States is taking a series of steps to sharpen this choice."
Speaking at a news conference at the State Department in Washington, she said the US expected "additional sanctions" by other countries in the days ahead.
The US also named Iran a "primary money laundering concern".
The US Treasury Secretary, Timothy Geithner, who appeared with Mrs Clinton at the news conference, warned that international banks should be wary of doing any business with Iran that might aid its nuclear programme.
"Financial institutions around the world should think hard about the risks of doing business with Iran," Mr Geithner said.
US firms were also warned against dealing with Iranian banks.
President Barack Obama said in a written statement that the United States had identified "the entire Iranian banking sector - including the Central Bank of Iran - as a threat to governments or financial institutions that do business with Iranian banks".
"As long as Iran continues down this dangerous path, the United States will continue to find ways, both in concert with our partners and through our own actions, to isolate and increase the pressure upon the Iranian regime," he said.
The British Chancellor of the Exchequer, George Osborne, had earlier said that all UK credit and financial institutions had to cease trading with Iran's banks from Monday afternoon.
"We believe that the Iranian regime's actions pose a significant threat to the UK's national security and the international community," he said.
"Today's announcement is a further step to preventing the Iranian regime from acquiring nuclear weapons."
It is the first time the UK has cut off a country's banking sector in this way.
The UN Security Council has already passed four rounds of sanctions against Iran for refusing to halt uranium enrichment.
Highly enriched uranium can be processed into nuclear weapons.
The US has already slapped sanctions on dozens of Iranian government agencies, officials and businesses over the nuclear programme.
Iran's deputy oil minister Abdolhossein Bayat said the new sanctions would not stop Iran exporting petrochemicals to the European Union, according to the semi-official Mehr news agency.
The agency said Iran had an 11% share of the European market.