"It's very important that we send this message that there is going to be a price to what Iran does. And that price will be increased isolation and heightened sanctions," the US under-secretary of state for political affairs, Nicholas Burns, told journalists ahead of a meeting with IAEA chief Mohamed ElBaradei.

"If Iran has not suspended its enrichment program in Natanz by a couple weeks' time, that's going to be a highly relevant factor" as it will show Tehran has not complied with UN Security Council resolutions, Burns added.

The Security Council has already passed two resolutions calling for sanctions if Iran does not fully suspend its enrichment and reprocessing activities and the United States is pushing for a third.

"Iran has chosen the route of sanctions," Burns said.

Ahead of his meeting with ElBaradei, he added that the US took issue with comments the director-general had made in the past "that would seem to indicate that sanctions might not work."

ElBaradei sparked controversy in the US when he told CNN on Sunday that he had no evidence that Iran was building nuclear weapons and emphasised the need for "creative diplomacy" rather than sanctions.

"I don't see any other solution than diplomacy and inspections," ElBaradei said.

Burns repeated calls for the Security Council to pass a sanctions resolution on Tehran as soon as possible.

He also called for the European Union to impose further sanctions and for major trading partners to cut ties with Iran.

The five permanent Security Council members -- the US, China, Russia, France and Britain -- and Germany will meet Friday in London to discuss strengthening UN sanctions against Tehran.

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