Iran on Tuesday rejected as "unacceptable" a demand by U.S. President Barack Obama that Tehran freeze its sensitive nuclear activities for at least 10 years, the semi-official Fars news agency reported.

"Obama's stance ... is expressed in unacceptable and threatening phrases ... Iran will not accept excessive and illogical demands," Foreign Minister Mohammad Javad Zarif was quoted as saying by Fars.

"Tehran will continue nuclear negotiations with the six powers," he added.

In an interview with Reuters on Monday, Obama said Iran must commit to a verifiable halt of at least 10 years on sensitive nuclear work for a landmark atomic deal to be reached between Tehran and six major powers.

The United States and its European allies fear Iran wants to develop atomic bomb capability. Tehran says its program is for peaceful nuclear energy only.

Iran and the United States, Britain, France, China, Russia and Germany have set a deadline of late March for a framework agreement and June for a comprehensive final settlement that would curb Iran's nuclear activity to ensure it cannot be put to bomb making in return for the lifting of international sanctions that have harmed Iran's economy.

Iran wants a swift lifting of the sanctions in any deal curbing its nuclear program -- one of the sticking points in high-level negotiations continuing in Switzerland this week.

Obama's comment about the time frame for a freeze represents one of the U.S. government's strongest signals yet of its red line for a successful agreement.