Talks in Vienna between Iran and six world powers over Tehran's nuclear program were postponed to next year.
British Foreign Secretary Philip Hammond said Monday's midnight deadline for a nuclear deal had been impossible to meet and it had therefore been extended to 1 July 2015. "It was not possible to get an agreement by the deadline, " Hammond said.
Iran and six powers - the United States, China, Russia, Britain, France plus Germany - have been negotiating intensively since February to turn an interim accord reached with the Islamic Republic a year ago into a lasting agreement.
The six nations want Tehran to scale back its nuclear program in exchange for a lifting of sanctions.
Quoting a Western diplomat close to the talks. the 'Reuters' news agency had earlier reported that negotiators were hoping that a deal on "political pieces" could be reached by March. "Some progress has been made," said a diplomat involved in the talks. "But we need to discuss some issues with our capitals. We will meet again before the new year. This is an ongoing process."
US Secretary of State John Kerry had earlier said in Vienna that "serious gaps" remained between them and Iranians. He met with Iranian Foreign Minister Mohammad Javad Zarif several times over the past few days in Geneva.
After 12 years of rising tensions, negotiators sought to reach a deal by Monday midnight aimed at easing fears that Tehran will develop nuclear weapons under the guise of developing energy for civilian use.
Iranian officials have refused to reduce the volume of uranium they are capable of enriching, a stand western officials say is unacceptable as this would potentially allow the country to amass enough fissile material for an atomic bomb in little time.
A second sticking point is the pace and sequencing of sanctions relief. Iran wants them terminated swiftly, not suspended and gradually scrapped, depending on the degree of Iranian compliance with the deal terms, as the West wants.
Another dispute is over the deal's duration: Western countries want it to be up to 20 years but Iran wants it much shorter.
