iranNEW YORK (AFP) - – Six major powers wrapped up what they said was a "very constructive" meeting on fresh UN sanctions against Iran over its nuclear program, but diplomats cautioned against expecting early adoption.

"We have just had a very constructive consultation," China's UN Ambassador Li Baodong told reporters as he emerged from the closed-door meeting held at the US mission to the United Nations.

"We now have a better understanding of each other's positions. We will continue these consultations."

The meeting brought together envoys from the five veto-wielding members of the UN Security Council -- the United States, Britain, France, Russia and China -- plus Germany.

The French, British and Russian ambassadors also described the meeting as "constructive."

Diplomats earlier said envoys of four Western powers were trying to enlist the support of their Russian and Chinese counterparts for a fourth set of UN sanctions aimed at persuading Tehran to halt its uranium enrichment program.

On the table was a US draft resolution outlining sanctions in five areas: arms embargo, energy, shipping, finance and targeted punitive measures against Iran's powerful Revolutionary Guards, a diplomat familiar with the discussions said.

But others said they anticipated drawn-out discussions on the text and hinted that a vote by the full 15-member council might not take place until June.

In Washington, the US State Department meanwhile expressed doubt Wednesday that new attempts at dialogue could convince Iran to change course on its nuclear program, but said nations had the right to try anyway.

"We would disagree with those who would say, you know, put this (sanctions) on hold because we still think this has promise," State Department spokesman Philip Crowley said of any new dialogue attempts.

But he stopped short of saying nations should give up on such efforts, saying it remained "the right of any country to try to convince Iran to change course."

His remarks came in response to questions about Turkey, a key US ally that has been reluctant to support new sanctions against Tehran, seeking instead to promote dialogue with the Iranian regime.

Like Brazil, which is also cool to new sanctions on Iran, Turkey is a non-permanent member of the Security Council.

"We started to negotiate on the basis of the (US) text," said French ambassador Gerard Araud of Wednesday's six-way talks. "All six countries are on board (for future negotiations). We are getting into the substance. We are moving forward."

Russian Ambassador Vitaly Churkin said the six nations would meet again on the issue "very soon."

The United States and its Western allies believe Tehran is using uranium enrichment as a cover to build nuclear weapons, a claim Iran denies.

In Washington earlier Wednesday, US Undersecretary of State for Political Affairs William Burns spoke of "a sense of urgency" in crafting a fourth UN sanctions resolution in the face of Iran's continued refusal to freeze enrichment.

"We are working aggressively to adopt concrete measures that will serve as a platform to strengthen and expand upon existing sanctions and target the power centers mostly likely to have an impact on Iran's strategic calculus," he told the Senate Armed Services Committee in Washington.

A top US military official said Wednesday that Iran could make enough highly enriched uranium for a nuclear bomb in one year, but would likely not have the know-how to complete a weapon for 3-5 years.

General James Cartwright, the number two uniformed US officer, told the Senate Armed Services he was making "a historical estimate" not specific to Tehran and underlined that he could not predict "what problems they will encounter."

And Lieutenant General Ronald Burgess, the head of the US Defense Intelligence Agency, said Washington has not determined that Iran has decided to develop highly-enriched uranium.

The proposed US draft, which incorporates comments from Washington's European allies, includes sanctions targeting Iran's energy, arms, shipping and financial sectors, sources said.

But diplomats said Russia and China were opposed to any ban on investments in the energy sector and were likely to resist measures that would authorize seizures of cargo suspected of containing materials linked to Iran's nuclear activities.

The US draft would also expand sanctions against Iran's elite Revolutionary Guard Corps, a pillar of the regime and the driving force behind its controversial nuclear program.