Iran and six world powers ramped up the pace of negotiations on Tuesday ahead of a deadline for a preliminary nuclear deal that was less than 24 hours away, as both sides warned it was crucial to overcome differences that could wreck an agreement.

For nearly a week the United States, Britain, France, Germany, Russia and China have been trying to break an impasse in negotiations aimed at stopping Iran from having the capacity to develop a nuclear bomb, in exchange for an easing of international sanctions that are crippling its economy.

But disagreements on enrichment research and the pace of lifting sanctions threatened to scupper a deal that could end a 12-year standoff between Iran and the West over Tehran's nuclear ambitions and reduce the risk of another Middle East war.