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.

Kerry postponed his flight from Geneva to other meetings in Sofia, Bulgaria, to resume meeting with Zarif late on Wednesday.

Ahead of the first session, Zarif said his talks with Kerry would show that both sides understood the importance of picking up the pace.

"I think it's important," he said. "I think it will show the readiness of the two parties to move forward to speed up the process."

Zarif was coy when asked if he thought the deadline could be met and what particular issues were most vexing.

"We'll see," he said. "All issues are hard until you resolve them and all issues are easy if you resolve them," he said. "I believe all of them are easy anyway."

Kerry and Zarif also took a walk together around Geneva. Asked how the talks were going, Kerry said: "We are working hard."

US officials said Kerry raised the cases of several US citizens detained by Iran. He said earlier this week that he was meeting Zarif to take stock of the negotiations.

"We are at a juncture where most of the issues are now getting fleshed out and understood," he said on Monday in India before he flew to Pakistan. He said he hoped to be able to "accelerate the process to make greater progress".

The US and its partners are hoping to clinch a deal with Iran that would set long-term limits on Iran's enrichment of uranium and other activity that could produce material for use in nuclear weapons. Iran says its program is solely for energy production and medical research purposes. It has agreed to some restrictions in exchange for billions of dollars in relief from US economic sanctions.

The negotiators are trying to turn an interim accord into a permanent deal that would address international concerns about Iran's nuclear program. In November, after failing to meet an earlier deadline, they set March as the target for a framework agreement and the end of June for a final pact.

After meeting with Zarif, Kerry will travel to Bulgaria, for talks with officials, and then to Paris, where he will see French officials, including President Francois Hollande, and pay his respects to the victims of last week's deadly terrorist attacks.

AP - AFP