Iran is arming the Taliban in Afghanistan, US Undersecretary of State Nicholas Burns said, accusing Tehran of fuelling insurrection around the Middle East and misjudging the international mood.

Paying a brief visit to France, Burns warned Iran that it would face increasingly tough sanctions if it refused to negotiate over its nuclear ambitions in the "next week or two."

Burns said Iran was a destabilising force and was funding extremists in the Palestinian territories, Lebanon and Iraq.

"Iran is now even transferring arms to the Taliban in Afghanistan. It is a country that is trying to flex its muscles, but in a way that is injurious to just about everyone else in the world. I think it is a major miscalculation," Burns said.

Earlier this month, US Defence Secretary Robert Gates said he could not link Tehran to a flow of weapons into Afghanistan and Afghan President Hamid Karzai hailed his country's relations with Iran as especially good.

Looking to up the pressure on Iran, Burns said Tehran would have to decide swiftly to talk to major world powers over its nuclear enrichment programme or see new sanctions imposed both inside and outside the context of the United Nations.

"We cannot imagine a stable, peaceful Middle East with Iran as a nuclear weapons power. I think all of us agree on that, including Russia and China," he said.

"The Iranians need to negotiate, they need to sit down in a civilised way or they'll find sanctions should they turn us down officially in the next week or two."

The West fears Iran is trying to develop nuclear weapons. Iran says its nuclear programme is for electricity generation and has refused to give up what it says is its right under an international treaty.

The United Nations has imposed two rounds of sanctions on Iran for not stopping enrichment and Burns said a third round would come "in a matter of weeks," with some countries likely to implement even harsher penalties than those envisaged by the Security Council.

"Iran has made a major miscalculation. Not only do they have Russia, China, Europe and the US sponsoring sanctions resolutions against them, they also have India, Indonesia, South Africa, Brazil, Egypt voting against Iran," he said. Reuters