US Secretary of State John Kerry (R) and Iranian Foreign Minister Javad Zarif (L) shake hands, as EU envoy Catherine Ashton watches, before a meeting in Vienna November 20, 2014. [Photo/Agencies]

VIENNA - A deadline for resolving a 12-year-old dispute over Iran's nuclear program may be extended from Monday until March because of sharp disagreements between Tehran and world powers, officials close to the talks said on Thursday.

US Secretary of State John Kerry was due to meet Iranian Foreign Minister Mohammad Javad Zarif with European Union envoy Catherine Ashton to try to break the stalemate, but talk of an extension prompted calls for tougher sanctions in Washington.

The United States and its allies had hoped this week's talks in Vienna would be the culmination of months of difficult diplomacy between Iran and the United States, Britain, France, Germany, Russia and China on a comprehensive agreement.

The aim is to remove sanctions on Tehran in exchange for curbs on its atomic program. But the talks have long been deadlocked. The timing for lifting sanctions and future scope of Iran's uranium enrichment are stumbling blocks.

"Important points of difference remain," French Foreign Minister Laurent Fabius told a news conference in Paris with Kerry.

The UN nuclear chief, Yukiya Amano, on Thursday highlighted another hurdle: Iran has yet to explain suspected atomic bomb research to the UN nuclear agency, one of the six powers' conditions for lifting sanctions.

The latest round of talks between the six and Iran began on Tuesday and are likely to last right up to a self-imposed Nov 24 deadline for a final agreement.