Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff, said the US military was prepared for any eventuality in Iran, despite being stretched by wars in Iraq and Afghanistan. He pointed to potential resources in the Navy and Air Force.

Mullen said authorities in Tehran were "on a path that has strategic intent to develop nuclear weapons and have been for some time" - a charge Iran denies.

Tehran already has been hit with three rounds of UN sanctions for refusing to comply with demands that it halt sensitive nuclear activities. The United States and its allies have said it is time for a fourth round of sanctions, but diplomats say Russia and China are resisting.

President Barack Obama has offered Iran the possibility of deeper engagement with the United States if it cooperates in removing fears about its nuclear programme and in other issues. This reversed the policy of Obama's predecessor George W. Bush, who had advocated isolating and punishing Iran.

Obama had given Iran until the end of 2009 to respond to his overtures and to an offer from six major powers of economic and political incentives in exchange for a suspension of Iran's nuclear enrichment program. Iran ignored the deadline.

The powers negotiating with Iran are UN Security Council permanent members United States, Russia, China, France and Britain, plus Germany.