America last night said that Iran had offered an inadequate response to the UN's demand that it stop enriching uranium, raising tensions ahead of a deadline next week that could trigger sanctions against the country.

Tehran's refusal to stop enriching uranium in return for a package of trade incentives offered by the UN Security Council last month has drawn a cool response from European diplomats and the US. France last night called on the Iranians to stop their fuel cycle work immediately.

But statements from China and Russia stressing the need for patience and negotiations have indicated that Iran's response — a series of detailed questions and the offer of "serious talks" — might succeed in splitting the permanent members of the Security Council when the UN ultimatum expires on July 31.

America's verdict on the Iranian response came at the end of a day of telephone calls between President George Bush, Condoleezza Rice, the US Secretary of State, and Kofi Annan, the UN Secretary General, who is expected to visit Tehran next week against the wishes of the White House.

A spokesman for the US State Department said America was consulting its allies on the next steps to take in confronting Iran, but observed that Tehran had not addressed the central UN demand: that it stop enriching uranium, the first stage towards developing atomic weapons.

"We acknowledge that Iran considers its response as a serious offer, and we will review it," said Gonzalo Gallegos, the spokesman. "The response, however, falls short of the conditions set by the Security Council, which require the full and verifiable suspension of all enrichment-related and reprocessing activities."

The reply coincided with a report by US congressmen that challenged the American intelligence community, chastened by failures before the September 11 attacks and the war in Iraq, to come up with more aggressive assessments of Iran.

The report, issued by the Republican-dominated House Intelligence Committee, said America's intelligence community needed to gather more information about Iran's capabilities and intentions and "not shy away from provocative conclusions or bury disagreements in consensus assessments".

"Iran’s support of radical Islamists with weapons and money demonstrates in real terms the danger it poses to America and our allies," said Peter Hoekstra, the Republican chairman of the committee. "That Iran has announced it will continue its program of nuclear enrichment, in violation of a U.N. resolution, demonstrates it will not be satisfied until it poses a threat to the entire world."

The 29-page dossier, which featured inflammatory quotations from speeches by President Mahmoud Ahmadinejad, accused Iran of fomenting the insurgency in Iraq, providing Hezbollah with 10,000 rockets in its recent war with Israel and running a clandestine nuclear weapons programme.

Looking ahead to next week's UN deadline, Philippe Douste-Blazy, the French Foreign Minister, said that a return to the negotiating table hinged on Iran suspending its enrichment work: "Our hand is still extended. The Iranians know the rules of the game: first a suspension of sensitive nuclear activities."

But both China and Russia appeared to see more room for optimism in the Iranian response. China’s special envoy to the Middle East, Sun Bigan, said Beijing wanted a "peaceful settlement rather than resorting to force or threatening sanctions".

While Russia, which is building Iran’s first nuclear power plant, said that it wanted the UN's atomic watchdog, the IAEA, to lead the negotiating process, rather than the UN Security Council.

Since making its offer, Iran has said it is "waiting for a logical and just reaction from the major powers (but) we are ready to face any situation".

"If there is no international partner or a climate favourable to fair negotiations, we will react accordingly," said Gholamhossein Elham, a government spokesman, in Tehran. "Nothing has changed. We will continue our research activities, but we want understanding and dialogue."

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