A senior U.S. official said on Thursday Russia and China had been blocking tough U.N. sanctions against Iran for months but there would be a push to impose them if Tehran had not suspended nuclear activity within two weeks.

Nicholas Burns, U.S. Undersecretary of State for Political Affairs, was speaking before talks with the U.N. nuclear watchdog chief to check whether Iran was honoring an August deal to clarify past secret aspects of its nuclear program.

The five permanent powers on the U.N. Security Council plus Germany will meet in London on Friday to weigh broader sanctions amid increased saber-rattling between Iran and Washington stirring fears of war if diplomatic pressure fails.

Iran has defied three Council resolutions, two with modest sanctions attached, demanding it stop enriching uranium. Iran says it wants nuclear-generated electricity, while Western powers suspect its program is a disguised bid for atom bombs.

Burns said Iran had been given a grace period since the last resolution on March 24 to allow for talks conducted by the EU's foreign policy chief to bear fruit, but Iran had stuck to a refusal to suspend.

Russia and China have been effectively blocking a third resolution since then, he told reporters. Moscow and Beijing, two of the five veto-holders on the Council and major trade partners of Iran, have insisted on more time for diplomacy.

To preserve shaky unity, Western powers agreed in September to put off seeking harsher sanctions pending results of Iran's transparency pledge to the International Atomic Energy Agency (IAEA), which will issue a report in mid-November.

But Burns said a clean bill of health from the IAEA alone would not be enough to halt steps to stiffer economic sanctions.

Our judgment is that if Iran has not suspended in the next couple of weeks, that's not sufficient, it will remain a refusal to meet Security Council requirements. That will be a highly relevant factor for us, he said.

Our hope is the following: first, a third sanctions resolution will be passed as soon as possible. Second, we'd very much support seeing the EU go forward with (its own) sanctions. Third, major trading partners of Iran should reduce trade to show Iran that this is not business as usual.

Iran warned the United States on Wednesday it would find itself in a quagmire deeper than Iraq if it attacked the Islamic state.

U.S. President George W. Bush has suggested a nuclear-armed Iran could lead to World War Three. Washington says it wants a diplomatic solution but Burns said on Wednesday more tough-minded diplomacy was needed to make that work.

He said this should include European sanctions on Iran, which some large EU members are reluctant to pursue.

Russia says dialogue rather than punishment or talk of military action offers the best way to ease tension over Iran.