With Russia and China putting up fierce resistance to punitive measures against the Iranians, US Undersecretary of State Nicholas Burns stepped up the call for countries to act individually outside the UN framework.

"There are a lot of countries that trade with Iran that have billion-dollar trade relationships and they ought to begin to rethink those commercial trade relationships," he told a news briefing here.

"There are a lot of countries that allow the export of dual-use technologies, and the position of the United States is that should be prohibited. All countries should refrain from military sales and arm sales."

Burns, undersecretary for political affairs and the number three official in the State Department, singled out Russia's plans to sell Iran 29 TOR M1 mobile surface-to-air missile defence systems.

"We hope and we trust that that deal will not go forward because this is not time for business as usual with the Iranian government," he said.

Robert Joseph, undersecretary of state for arms control, said he toured the Gulf last week and held talks with regional officials on a variety of other possible measures against Iran.

Among them were efforts to intercept the transfer of sensitive materials and technologies, a crackdown on front companies and bank accounts used to aid Tehran, and greater cooperation in missile defense.

Burns spoke after holding meetings earlier this week with counterparts from Russia, China, Britain, France, Germany, Italy and Canada that produced no consensus on possible UN sanctions.

The UN Security Council has given the Islamic republic until next Friday to halt its uranium-enrichment activities but has not laid out any consequences if it refused to comply.

The United States is pressing for a threat of UN sanctions such as freezes on the assets of Iranian leaders or travel restrictions. Yet Burns recognized the outcome was far from certain.

"The diplomacy here is very challenging. It is likely to extend some time into the future," he said. "I can't predict where the Security Council will be a month from today."

"It's also important for countries to know that, if for any reason the Security Council can't succeed, there will be an effort made, and there are avenues available to us to apply the kind of pressure that we need to apply."

Burns said political directors of the UN Security Council's five permanent members -- the United States, Russia, China, Britain and France -- would likely meet May 2 in Paris to thrash out a strategy.

He said Iran would also be "the leading issue" when foreign ministers of the Group of Eight industrialized powers meet in Moscow in June, and G-8 leaders hold their summit in St. Petersburg the following month.

The United States has been ratcheting up its diplomatic effort against Tehran since Iran announced on April 11 it had taken a major step forward in its nuclear program by enriching small amounts of uranium.

Joseph said the Iranians were "very close" to a point of no return in the West's ability to curb their march towards mastery of the sensitive nuclear fuel cycle.

"It's fair to say, I believe, that the Iranians have put both feet on the accelerator," Joseph said. "They're moving very quickly to establish new realities on the ground associated with their nuclear program."

He said he had little reason to doubt the Iranians' claim they had converted enough uranium for 110 tons of UF-6 gas for enrichment in centrifuges, and this would mean they had accumulated sufficient material for more than 10 bombs.

Tehran's assertion it was able to operate a cascade of 164 centrifuges and enrich uranium to 3.5 percent would mean they were "well on your way" to producing industrial-size volumes of weapons-grade material, Joseph said.

Iran, which insists its nuclear program is strictly for peaceful purposes, has announced plans to install 3,000 centrifuges within a year and ultimately run some 54,000 centrifuges.

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