"In the next few weeks, we want to extend the measures and sanctions to include investment in liquefied natural gas. I believe that sends another signal to the regime that what is happening is unacceptable," said Brown.

The prime minister's comments came as he met at the White House with US President George W. Bush, who said anyone who thinks that Iran's uranium enrichment program has no military applications "is, in my judgment, naive."

"Our position is clear: That we're going to work together along with other nations to make it abundantly clear to the Iranian regime that they must not have the capability of developing a nuclear weapon," said Bush.

"Gordon Brown seriously sees the threat, as do I. And now is the time to confront the threat. And I believe we can solve the problem diplomatically," the president said during a joint appearance in the Rose Garden.

Iran has defied UN demands to freeze its uranium enrichment program -- a process that can be a key step towards obtaining nuclear weapons -- and spurned three rounds of UN sanctions meant to bring it in line.

"I'm talking to other European leaders about how we can extend European sanctions against Iran over the next period of time, and to ensure that what sanctions are taken are effectively implemented," said Brown.

Brown said Britain, the United States, would work with partners France, Germany as well as Russia and China "to make sure that Iran recognizes that it cannot ignore the international community and its obligations ... with impunity."

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