"In the absence to a positive response to the generous offer that we provided for our incentives package, we think that the allies will have no choice but to take further measures that would be punitive, given that we don't have a decent and responsive statement from the Iranians," said Perino.

Perino spoke on the jet taking President George W. Bush from South Korea to Thailand as part of an Asian tour.

US media reports have said Washington and its allies will press for tough UN Security Council sanctions against Iran's nuclear drive following its latest letter.

The United States, Britain, China, France, Germany and Russia were to hold telephone talks at 1115 GMT on Iran's latest written response to an offer of economic and technical incentives.

"I think that the Iranians have long stalled on responding to the allies, so I think the most important thing we can do is let the political directors have their conference call and decide on their next step," Perino told reporters.