US Secretary of State Hillary on Thursday warned Iranian President Mahmoud Ahmadinejad that he will fail if he tries to disrupt next week's UN nuclear talks.

Ahmadinejad has asked for a visa to travel to New York to lead his country's delegation to the nuclear Non-Proliferation Treaty (NPT) review conference, something US officials say they expect to approve.

Clinton, who will attend the gathering, said the Obama administration aims to use the conference to push for deeper nuclear disarmament, check the spread of atomic weapons and promote the peaceful use of civilian nuclear energy.

"That is the purpose of our going to New York," the chief US diplomat told reporters in Washington as she stood next to Polish Foreign Minister Radoslaw Sikorski, with whom she had discussed the conference and other issues.

"I don't know the purpose that Iran sees, because their record of violations of the non-proliferation obligations that they assumed as a signatory to the NPT is absolutely indisputable," said Clinton.

She recalled that the Islamic Republic was exposed late last year for trying build a covert uranium enrichment plant near the holy city of Qom and that its atomic work has been found to violate UN Security Council resolutions.

"So if President Ahmadinejad wants to come and announce that Iran will abide by their non-proliferation requirements under the NPT, that would be very good news indeed and we would welcome that," the secretary of state said.

But if he believes "he can somehow divert attention from this very important global effort or cause confusion that might possibly throw into doubt what Iran has been up to... I don't believe he will have a particularly receptive audience," she said.

Iran is seen as a test case for the treaty, both because it resists UN demands to stop enriching uranium and because an Iranian bomb could set off an atomic arms race in the Middle East.

The United States charges that Iran is secretly developing atomic weapons but Iran says its program is to generate electricity.

Clinton's spokesman Philip Crowley said earlier that the State Department is still processing visas for the Iranian delegates asking to attend the conference.

He also told reporters that "a face-to-face meeting between a US diplomat and an Iranian diplomat is highly unlikely," when asked if such direct talks would take place at UN headquarters where 189 nations will be represented.

And Crowley said Clinton spoke by telephone earlier Thursday with Chinese State Councilor Dai Bingguo about the US-led push for tougher UN Security Council sanctions against Iran over its refusal to stop uranium enrichment.

China -- one of the five permanent, veto-wielding members of the 15-member council -- as well as current but temporary members Brazil, Turkey and Lebanon have been reluctant to embrace biting sanctions.

Iran has led an intensive worldwide diplomatic campaign to drive a wedge between supporters and opponents of sanctions on the council.

Top US arms control negotiator Ellen Tauscher has meanwhile set modest ambitions for the NPT review conference saying it would be successful even without a consensus final document.

"A final document can easily be blocked by the extreme agendas of a few," Tauscher said.

"A review conference that reaffirms the basic bargain at the heart of the treaty and demonstrates broad support for strengthening nonproliferation measures should be considered success," she added.

"A draft final document or a streamlined action plan that draws the support of all but a few outliers would meet this definition of success," she said.