"We have just had a very constructive consultation," Li Baodong, China's ambassador to the UN, told reporters as he emerged from the more than three-hour closed-door meeting.

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

Vitaly Churkin, Russia's ambassador, said the six nations would meet again "very soon".

Diplomatic consultation

On the table at the closed-door meeting was a draft UN resolution circulated by the US in January, with some changes proposed by Britain, France and Germany.

in depth

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  Countdown: The Iran/Israel arms race
  Timeline: Iran's nuclear programme

The draft resolution focuses on five areas: strengthening the existing arms embargo, targeting Iran's powerful Revolutionary Guards, imposing new sanctions on its energy sector, and strengthening sanctions on its shipping and finance sectors, a UN diplomat familiar with the talks said.

It would also add new names of individuals and entities to a list of those subject to an asset freeze and travel ban for their proliferation-related activities.

Although Barack Obama, the US president, has called for UN Security Council action in weeks, a UN diplomat, speaking on condition of anonymity, said negotiations will be tough and it is much more likely that a resolution will not be put to a vote until June.

After months of behind-the-scenes consultations, China agreed in late March to take part in talks on a fourth sanctions resolution.

Obama said on Tuesday that Hu Jintao, the Chinese president, had assured him that China would participate in drafting sessions on strong sanctions.

But Iran expressed doubts later on Tuesday that China, which relies on Iran for 11 per cent of its energy needs and last year became Tehran's biggest trading partner, would back the US push for new sanctions.

'Dual-track plan'

Jiang Yu, a Chinese foreign ministry spokeswoman, said in Beijing on Tuesday that China supports a "dual-track strategy", combining diplomacy with the possibility of international sanctions against Iran, but the country believes "pressure and sanctions cannot fundamentally solve it".

"I do not favour paralysing, crippling sanctions that make people suffer"

Dmitry Medvedev, Russian president

Dmitry Medvedev, Russia's president, has also sought to temper US hopes for biting sanctions, conceding this week that "if nothing happens, we will have to deal with sanctions".

He also warned: "I do not favour paralysing, crippling sanctions that make people suffer."

However, the US has cast doubt on the chances of diplomacy convincing Iran to abandon it nuclear enrichment programme.

"We would disagree with those who would say, you know, put this [sanctions] on hold because we still think this has promise," Philip Crowley, the US state department spokesman, said on Wednesday.

Tehran has insisted that its nuclear programme is simply to meet its civilian energy needs, but Washington and its allies say that they fear it could be a cover for the production of atomic weapons.

A senior US military official told the US senate armed services committee on 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 at least three years.

General James Cartwright, the second-most senior uniformed US officer, said he was making "a historical estimate" not specific to Iran and underlined that he could not predict "what problems they will encounter".

Bushehr 'on schedule'

Meanwhile on Wednesday, the head of Russia's state nuclear corporation said a nuclear reactor it was building at Iran's Bushehr plant is scheduled to open in August.

"The launch is scheduled for August. We're on schedule," Sergei Kiriyenko, head of Russia's Rosatom Corp, told reporters on a visit to Argentina.

Hillary Clinton, the US secretary of state, has criticised Russia's plans to start up the nuclear power station, saying it was "premature" without further assurances on Tehran's nuclear programme.

But Kiriyenko said: "Bushehr doesn't threaten the regime of non-proliferation in any way. No one has any concerns about Bushehr."

Russia agreed to build the 1,000-megawatt reactor at Bushehr 15 years ago but delays have haunted the $1bn project and diplomats say Moscow has used it as a lever in relations with Tehran.