Iranian Foreign Minister Mohammad Javad Zarif says gaps remain in the talks with the United States and other major powers aimed at securing a lasting deal on Tehran's nuclear program.
Zarif and U.S. Secretary of State John Kerry held nearly five hours of talks in the Swiss city of Lausanne on March 16, before the Iranian delegation headed to Brussels for meetings with European ministers.
After the meeting in Switzerland, Zarif said, "The solutions are within reach but we still have gaps in some areas."
He added that talks needed to continue this week to see what could be achieved.
A senior U.S. official was quoted as saying Iran "still has to make some very tough and necessary choices to address the significant concerns that remain about its nuclear program."
The official told reporters that it was not clear if an end-of-March deadline for a framework agreement that would curb Iran's nuclear ambitions in exchange for relief from international sanctions could be met.
A final agreement is due by June 30.
The foreign ministers of Britain Germany, France, and EU foreign-policy chief Federica Mogherini met with Zarif later on March 16 in Brussels.
European Union Foreign Policy Chief Federica Mogherini told reporters the talks in Brussels had been helpful.
"We discussed all the remaining open gaps and the way forward," she said.
Earlier, Mogherini said "there are still some gaps that need to be tackled” while adding that “my hope is that tonight our European side's contribution can help.”
The Iranian delegation is now due to return to Lausanne for more talks with the Americans.
British Foreign Minister Philip Hammond said that negotiations still had some way to go before a deal could be reached with Tehran.
"We are closer than we were but we've still got a long way to go," he told reporters in Brussels. "There are areas where we've made progress, areas where we have yet to make any progress."
Echoing Hammond's comment, French Foreign Minister Laurent Fabius said: "We want an agreement, but only if the agreement is very solid. There has been progress but important points remain which are not resolved," he said.
German Foreign Minister Steinmeier said that after "more than 10 years of negotiations, we should seize this opportunity" to solve the dispute over Iran's nuclear program.
Kerry said in Egypt on March 15 that most of the problems holding back an agreement were "political" rather than technical.
He didn't elaborate on the issue, but experts say the political matters include the levels of inspections by international monitors and how quickly the international community would scale back the tough economic sanctions against Iran under a deal.
Kerry said Washington's aim was to "get the right deal."
He said that Tehran "to its credit, has thus far lived up to every part of the agreement we made over a year ago."
But he added that "important gaps" remained and Iran must make "important choices" in order to move forward.
Technical matters expected to be discussed before a deal could be made include how many uranium-enriching centrifuges Iran can keep, the types of centrifuges it can have, and how much plutonium Tehran could produce in a planned heavy-water reactor.
Officials involved in the talks said Western powers were seeking concessions from Iran after the United States and European countries involved in the talks indicated a willingness to compromise on suspending UN sanctions.
The Reuters news agency reported last week that the six powers and Iran started discussing a possible draft UN resolution to endorse any future agreement and to address the lifting of UN sanctions.
Western officials said the UN sanctions could be eased quickly in the event an agreement was reached.
That was seen as a major concession on the part of the United States, which had long insisted that UN sanctions should remain in place for several years after a deal was signed, while U.S. and European sanctions could be removed more swiftly.
Iran wants the UN, EU, and U.S. sanctions lifted at the same time.
Tehran and the six world powers have been negotiating for years and reached an interim agreement in November 2014, but two self-imposed deadlines for a broader deal passed last year without agreement.
Tehran insists that its nuclear program is only for peaceful purposes, but other countries suspect it might harbor covert ambitions to build nuclear weapons.
Kerry has repeatedly said that "no deal is better than a bad deal."
Iran's Supreme Leader Ayatollah Khamenei has said that no agreement is better than an agreement that runs contrary to Iran's interests.
With reporting by Reuters, IRNA, AP, and AFP