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Iran faces its toughest sanctions for failing to comply with UN demands over its nuclear enrichment, the US says.

The Security Council is set to vote on a fourth round of sanctions to tighten financial curbs on Iran, while expanding a limited arms embargo.

US Secretary of State Hillary Clinton said they were "the most significant sanctions that Iran has ever faced".

But Iran's president warned his country would not agree to nuclear talks if the sanctions were imposed.

Mahmoud Ahmadinejad also said that a nuclear fuel-swap deal brokered by Turkey and Brazil was an offer that would not be repeated.

Iran agreed in May to deposit 1,200kg of low-enriched uranium with Turkey, in return for reactor fuel, but the US and its Western allies said the agreement was too little too late.

They fear Tehran is secretly trying to build a nuclear bomb. Iran says its programme is aimed solely at peaceful energy use.