EU foreign policy chief Catherine Ashton met senior officials from world powers on Monday to prepare for talks in Moscow this month on Tehran's contested nuclear drive.

Ashton and officials from the so-called P5+1 group -- Britain, China, France, Russia and the United States plus Germany -- met in a Strasbourg hotel "to prepare the ground for the negotiations in Moscow," an EU spokesman said.

Moscow on June 18-19 will host a third round of negotiations between Iran and the global powers that up until now have failed to yield results in efforts to curb Tehran's nuclear activities.

Ashton represents the P5+1 in dealings with Iran, which the West suspects is trying to build the atomic bomb, charges long denied by Tehran.

Also in Strasbourg was the US lead negotiator, Under-Secretary of State for Political Affairs Wendy Sherman, the State Department said.

"As part of our dual-track policy to prevent Iran from acquiring a nuclear weapon, the United States remains united with other P5+1 partners in our commitment to serious preparations for the Moscow round of talks and to enabling the diplomatic track to succeed," it said in a statement.

"Iran has the opportunity to begin addressing international concerns over its nuclear programme by coming to Moscow prepared to take concrete steps in response to the proposals presented in Baghdad."

The Moscow round takes place just two weeks before the European Union imposes a full embargo on Iranian oil.

The Strasbourg talks follow complaints from Tehran about what it says is a lack of willingness by the P5+1 to engage.

Ali Bagheri, deputy to Iran's top negotiator Saeed Jalili, said in a letter to Ashton's deputy Helga Schmid that he was "surprised" by issues she was raising in correspondence with him.

He also complained that preparatory groundwork by experts from both sides was needed before the talks.

The Moscow round follows two earlier unproductive meetings since early April, in Istanbul and in Baghdad.

But Ashton's spokesman Michael Mann told AFP the EU was "surprised" by the Iranian letters.

The group "is always ready to have discussions on substance, but Iran continues to put the focus on procedural aspects," he said.

He said the P5+1 had explained elements of its package during "long" meetings between Schmid and Bagheri in Geneva before Baghdad, and "handed over a non-paper in Baghdad on details of the proposal."

"At that time Iran was not ready to engage in experts' discussions on the basis of our balanced package," he said.

"There were several phone calls and exchanges of letter at deputy level during past two weeks, but Iran has not been willing to advance issues on substance," Mann said.

He said Ashton planned to talk to Jalili "to get political commitment for the process to move ahead."