WASHINGTON (Reuters) - The leaders of the United States and Britain on Thursday pledged a united effort to prevent Iran from developing a nuclear weapon, possibly by expanding sanctions against Tehran.

“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,” President George W. Bush said at a Rose Garden news conference with Prime Minister Gordon Brown.

His comments came after Mohamed ElBaradei, director-general of the International Atomic Energy Agency, said Iran’s progress in developing uranium enrichment is slow and that recent additions to its nuclear fuel production complex have only been older-model centrifuges.

“If they learn how to enrich, it is knowledge which can be used to develop a nuclear weapon,” Bush said.

The United States has been leading efforts at the United Nations to pressure Iran to halt its nuclear programme, which Tehran says is only for civilian purposes.

“Iran continues to defy the will of the international community. And we are agreed on the need to strengthen the sanctions regime and ensure that these sanctions are effectively implemented,” Brown said.

The United States has repeatedly said it is pursuing a solution through diplomatic efforts. “And now is the time to confront the threat. And I believe we can solve the problem diplomatically,” Bush said.

Brown said he was talking with other European leaders about how to extend sanctions against Iran. “I make no apology for saying that we will extend sanctions, where possible, on Iran,” he said.

“So 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,” Brown said.

A senior U.S. Treasury official in Washington said U.S. sanctions banning transactions with Iran’s central bank are among the options in the Bush administration’s “toolbox” to pressure Iran to limit its nuclear programme.