Six world powers that are poised to restart stalled talks with Iran sought on Wednesday to agree a unified stance on the Islamic Republic's nuclear programme, diplomats at the United Nations' nuclear agency said.

The United States and its Western allies - which have led international pressure on Tehran - initially pursued a resolution by the U.N. agency's board of governors to rebuke Iran over what they see as its failure to answer mounting concerns of a disguised bid to develop nuclear arms capability.

But diplomats said Russia and China - which are less keen on tightening sanctions - saw no need for a new resolution so soon after one was passed at the last 35-nation board meeting of the International Atomic Energy Agency (IAEA) in November.

Instead, the focus is now on crafting a joint statement to be delivered at the current board meeting, which took the unusual step to adjourn until Thursday to give more time for big power envoys to consult with each other and their capitals.

One senior Western diplomat played down any suggestion of major differences between the four Western states - the United States, France, Britain and Germany - and Russia and China.

It was nothing that we can't resolve, the envoy said.

A joint statement would underline the importance of the powers' upcoming talks with Iran and also urge it to cooperate with IAEA inspectors after two recent rounds of largely fruitless meetings in Tehran, another Western diplomat said.

The Western camp would want to see relatively tough language on Iran to pressure it to cooperate with the IAEA while China and Russia seek a milder statement to help foster a constructive atmosphere for more talks, analysts say.

A European diplomat, not from any of the six powers, said interrupting the meeting for a whole day suggested more fundamental differences between the four Western states and Russia and China on how best to approach the Iran issue.

He warned that failure to agree on a joint statement could affect the powers' talks with Iran. At past meetings the big powers have been able to paper over their differences.

Everybody is waiting for them to agree on this text, he said. It is four versus two.

ENRICHMENT HALT?

EU foreign policy chief Catherine Ashton, who would lead future talks with Iran on behalf of the six powers, announced on Tuesday there would be an attempt to revive the talks - stalled for more than a year, aimed at allaying suspicions that Iran is developing nuclear weapons.

A date and venue have yet to be agreed for the talks, proposed by Iran after a year's diplomatic standstill that has increased fears of a slide into a new Middle East war.

Russia and China, which have notable commercial ties to Iran, have backed four rounds of U.N. sanctions on the major oil producer since 2006 over its refusal to curb nuclear activity that can have military and civilian purposes alike.

But both Moscow and Beijing have criticised the United States and the European Union for imposing much harsher unilateral punitive steps on Iran, suggesting they could be counterproductive.

Russian Deputy Foreign Minister Sergei Ryabkov has said fresh talks with Iran should address a proposal by president-elect Vladimir Putin for the powers to formally recognise its right to enrich uranium, Iran to submit its programme to full IAEA supervision, and international sanctions to be lifted.

Some Western experts say it is unrealistic to expect Iran to halt all enrichment - activity which can have both civilian and military purposes - and that the powers would likely need to compromise on this point for a deal.

In return, they say, Iran must agree to much more intrusive U.N. inspections to ensure there is no secret nuclear work.

But a letter from Ashton to Iran this week said a settlement between us would involve the full implementation by Iran of United Nations Security Council resolutions, under which Tehran should suspend all enrichment-related activities.

(Reporting by Fredrik Dahl; Editing by Mark Heinrich)

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