The head of the UN nuclear watchdog pressed Iran on Monday to speed up cooperation with the agency's probe into Tehran's atomic programme.
"This process cannot continue indefinitely. It is not an endless process," said Yukiya Amano, director-general of the International Atomic Energy Agency.  
While Iran is negotiating limits on its programme with world powers, the IAEA has been pressing leaders in Tehran for years to address allegations that prior to 2003, and possibly since, they conducted research into developing nuclear weapons.
Iran has consistently rejected the claims, set out in a major IAEA report in 2011, as baseless.
However, Iran had agreed to answer agency inquiries about alleged explosive testing and research into nuclear bomb making by August, but the responses have yet to be given.  
"The agency remains ready to accelerate the resolution of all outstanding issues," Amano said. However, "this requires increased cooperation by Iran and the timely provision of access to all relevant information."
Amano met with Iranian leaders in February, and last week Tehran's deputy foreign minister Abbas Araghchi pledged his country would "move faster and in a better sense" with the probe.
The P5+1 group of world powers have until March 31 to reach a framework for a deal, which would then be firmed up and officially signed on June 30.
US Secretary of State John Kerry is due to meet his Iranian counterpart Mohammad Javad Zarif in the Swiss lakeside town of Montreux later Monday for talks on the agreement.