This week, Britain, France and Germany are to meet again with the Iranians in a bid to persuade Tehran to give up operations linked to the enrichment of uranium, which the so-called EU-3 and Washington believe could be used to manufacture a nuclear weapon.

"The bar for Iran must be set very high because of its history of deception," US under secretary of state for political affairs Nicholas Burns told the Senate Foreign Relations Committee on Thursday.

But, he said, "We see no sign Iran has made the necessary strategic decision to abandon its nuclear weapons program."

In March, Washington gave Europeans the go-ahead to propose supporting Tehran in its bid to join the World Trade Organization and to provide spare parts for civil aviation, all in exchange for opening talks aimed at ending Iranian efforts to build an atomic bomb.

"Iran's leaders will never comply with those demands unless the United States and the EU can offer the Iranian government far more incentives than they have been so far prepared to put on the table," said Geoffrey Kemp, an analyst at the Nixon Center who also appeared before the Senate panel on Thursday.

According to Kemp, "Absent some fundamental change in the Iranian leadership, combined with a willingness on the part of the Bush administration to take big risks, the United States is on course for a serious crisis with Iran at some point in the coming months."

A US official, speaking on condition of anonymity, recently said negotiations are currently in a "tactical" phase due to June 17 presidential elections in Iran. Amid the political uncertainty of the vote, it is difficult to know what will happen in the long term, the official said, welcoming the so-called EU-3's firmness with Iran.

Tehran, meanwhile, has made it clear it is looking to resume certain enrichment activities that are incompatible with the agreement on stopping nuclear activities, he said.

The signal given to Tehran by the three European nations is that the agreement must be respected and that talks must continue, the official stressed, adding that some hope is being kept alive here that the Iranians will agree to the European's requests.

The EU-3 and the United States, however, are using somewhat different weapons in approaching the negotiations.

"The Europeans have a great deal to offer Iran economically," said Gary Milhollin, director of the Wisconsin Project on Nuclear Arms Control at the University of Wisconsin.

"Europe, unlike the United States, has active commercial ties to Iran and had been negotiating a trade agreement with Iran before the present nuclear crisis erupted in 2003," he noted.

According to Milhollin: "The promise of future benefits in exchange for cooperation is the main thing Europe has to offer," with their denial the primary threat Europe has on its side.

Despite this, Milhollin said Tehran wants to develop nuclear weapons for national security reasons, partly to boost its own strategic position in the region.

However, "Only the United States is capable of providing Iran with adequate security assurances," Milhollin said, adding: "It should start thinking about how to do so."

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