"We would prefer that the Iranian regime follow through on the opportunity to engage," said Ellen Tauscher, Under Secretary for Arms Control and International Security at the US State Department.

Tehran "asked for engagement with the United States. It has it. Now what is it going to do? Is it going to stand up and say that they're going to take our deal ... or are they going to use some other flimsy excuse to duck," she said.

"If persuasion doesn't work, pressure is going to have to be the next line of action," Tauscher warned, suggesting a further round of sanctions against Iran.

Speaking at the Halifax International Security Forum, she said the United States continues to have "significant" concerns about Iran's nuclear ambitions.

But, she added, "I don't believe (military action against Iran) is on the table now."

Earlier, Ali Asghar Soltanieh, envoy to the International Atomic Energy Agency (IAEA), reiterated that Tehran is ready for further talks on supplying fuel for the internationally supervised reactor in the capital.

Meanwhile, a commander of the elite Revolutionary Guards said on Sunday that its air defenses would annihilate Israeli war planes if they attacked.

He spoke at the start of five days of maneuvers aimed at honing a response to any assault on Iran's nuclear facilities.

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