Iran and major powers remain "pretty far" from a deal aimed at preventing Tehran getting nuclear weapons, a European diplomat in crunch talks in Switzerland said Thursday while hitting out at Tehran's negotiating tactics.

"I think we are pretty far from a deal," the negotiator said on condition of anonymity. "I really don't believe that we are going to get a (framework) deal on Friday night."

"The Iranians go back, go forward, it changes every day," the diplomat said in Lausanne.

US Secretary of State John Kerry, his Iranian counterpart and negotiators from Russia, China, Britain, France, Germany and the EU are racing to meet a March 31 deadline for the outlines of a deal.

Such an accord, due to be finalised by July, is meant to convince the world that Iran will not develop nuclear weapons under the guise of its civilian programme -- an aim it denies having.

The mooted accord is however highly complex and would involve Iran scaling down its nuclear activities in exchange for staggered relief from painful sanctions.