World powers reaffirmed their commitment Thursday to reaching agreement with Iran on its contested nuclear programme by a July 20 deadline, after Tehran lashed out at "excessive demands" made by them.

Officials from Britain, China, France, Russia and the United States -- the five permanent members of the UN Security Council -- plus Germany met European Union foreign affairs head Catherine Ashton for "an intense day of preparation" for talks next week with Iran.

"The meeting affirmed the determination of the (group) to reach a comprehensive agreement by July 20," Ashton's spokesman Michael Mann said.

The next round takes place Wednesday in Vienna, led by Ashton and Iranian Foreign Minister Mohammad Javad Zarif, after a difficult fifth round there last week.

Earlier Thursday, Zarif said that while Tehran had made "rational proposals," the demands of the P5+1 group risked sinking the negotiations.

"Iran is ready for a resolution and made rational proposals," the official IRNA news agency quoted Zarif as saying.

"But the excessive demands of the other party could prevent an agreement," he said.

"At that time the world will know who was responsible for the deadlock in the nuclear negotiations."

Zarif had said last week that both sides had yet to find common ground on the main issues.

The P5+1 suspect Iran's programme is meant to produce nuclear weapons but Tehran insists it is purely for peaceful purposes.