PARIS (Reuters) - European Union foreign policy chief Federica Mogherini said on Sunday it was not impossible to get a deal between world powers and Iran over its nuclear program but said tough days lay ahead as a self-imposed June 30 deadline nears.

France's foreign minister said on Saturday an accord remained elusive due to disagreements over fundamental issues, while the U.S. and Iranian top diplomats said hard work was still needed for what could be their final negotiations to bridge significant differences.

"If a few days more are needed we will take them," Mogherini told reporters on arrival in Vienna.

"It is going to be tough... but not impossible. It is a matter of political will," she said before meeting the U.S., British, German and French foreign ministers.

The talks also include Russia and China.

The deadline for a deal under which Iran would cut back its nuclear program in exchange for relief from economic sanctions, expires on Tuesday.

Diplomats have said there is little chance of an accord by the deadline and expect the talks to spill over into July.

The main differences are on the pace and timing of sanctions relief for Iran in return for its steps to restrain its nuclear program and on the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.

U.S. and European negotiators also want to ensure there is a mechanism for restoring U.S., European Union and United Nations sanctions if Tehran fails to meet its commitments under any future accord aimed at ending a 12-year nuclear standoff between Iran and the West.

"I can’t tell you how long it will be. What I can tell you that is that we don’t want to have an extension of the talks. There will be no extension," said a senior Western diplomat.

The United States, Israel and some Western nations fear that Iran has been trying to develop a nuclear weapons capability but Tehran says its program is for peaceful purposes only.

(Additional reporting by Parisa Hafezi, Arshad Mohammed and Shadia Nasrallah; Editing by Angus MacSwan)