Top US and EU leaders urged Iran on Thursday to help ensure "progress" in the next round of talks over its nuclear program, even as moves to boost UN inspections of Iranian sites failed.

US Secretary of State John Kerry cautioned the next negotiations, due on February 26 in Almaty, Kazakhstan, "can only make progress if the Iranians come to the table determined to make and discuss real offers."

And he warned as he met with UN chief Ban Ki-moon that the United States was determined not to get trapped in "a delay-after-delay process.""Countries that have peaceful programs do not have problems proving to people that they are peaceful," Kerry told reporters."I think it is incumbent on the Iranians to prove that they are prepared to meet our willingness... to be open to a diplomatic resolution."

It took weeks of negotiations to agree on a date and venue for the next talks aimed at getting Tehran to rein in its nuclear enrichment program between the world powers, known as the P5+1, and Iran.Ban also expressed hope that the P5+1 meetings would "bring fruitful progress."

And EU foreign policy chief Catherine Ashton, who met with Kerry separately Thursday at the State Department, said: "I always look for success... and I will do my best on behalf of the P5+1.""It is important that we continue to track and make our efforts successful," she said.

But the chief UN atomic inspector said Thursday that separate talks with Iran had failed again to agree on enhanced inspections of its nuclear program."We had discussions on the structured approach document but could not finalize the document," Herman Nackaerts of the International Atomic Energy Agency told reporters after returning from Tehran."We will work hard now to resolve the remaining differences, but time is needed to reflect on the way forward."