The United States and Iran have expressed hope that nuclear talks can be accelerated in order to meet a March target for a framework agreement, as the countries' top diplomats met on Wednesday ahead of a resumption of full negotiations.

US Secretary of State John Kerry and Iranian Foreign Minister Mohammed Javad Zarif met for six hours on Wednesday, a day before negotiators from Iran, the five permanent members of the UN Security Council and Germany are to resume talks in Geneva.

"They had substantive meetings ... and they discussed a broad range of issues with a small group of staff from each side," a senior US State Department official said.