Iran and six world powers have agreed to a four-month extension of negotiations on a nuclear deal with Tehran after failing to meet a July 20 deadline due to "significant gaps" between the two sides, the European Union and Iran said on Saturday.

"There are still significant gaps on some core issues which will require more time and effort," EU foreign policy chief Catherine Ashton and Iranian Foreign Minister Mohammad Javad Zarif said in a joint statement.

Iran, the United States, Britain, France, Germany, Russia and China had set a July 20 deadline to complete a long-term agreement that would resolve the decade-old dispute over Tehran's nuclear ambitions. But diplomats said they were unable to overcome significant differences on major sticking points.

The White House said on Friday there was a "credible prospect for a comprehensive deal" with Iran over its nuclear program that made it necessary to extend talks.

"This extension will allow us to continue the negotiations while ensuring that the progress of Iran's nuclear program remains halted during the negotiations," White House spokesman Josh Earnest said in a statement.

"We have an opportunity to achieve a lasting, diplomatic solution that will resolve one of the most pressing national security issues of our time. We will not accept anything less than a comprehensive resolution that meets our objectives, which is why it is necessary for negotiations to continue."

Negotiations likely to restart in September

It has been clear for days that Iran and the six powers would miss the Sunday deadline to reach an accord due to disagreements on a number of key issues in the discussions. 

Among the issues dividing them are the permissible scope of Iran's nuclear fuel production capacity and how to address the country's suspected past atomic bomb research. The negotiations began in February in Vienna.

IRAN-NUCLEAR/KERRY

U.S. Secretary of State John Kerry said the 'vast majority' of Iran's frozen oil revenues will remain inaccessible during the four-month extension of nuclear talks. (Heinz-Peter Bader/Reuters)

The talks are taking place because of a preliminary agreement reached in Geneva in November 2013 that gave Iran limited sanctions relief in exchange for halting some nuclear activities and created time and space for the negotiation of a comprehensive deal to end the decade-long dispute.

The negotiations on a long-term deal were likely to resume in September, diplomats said. 

Iran will be allowed to access an additional $2.8 billion of its frozen assets during the four-month period but most sanctions against Tehran will remain in place, the United States said on Saturday.

"We will continue to suspend the sanctions we agreed to under the (preliminary agreement from November 2013) and will allow Iran access to $2.8 billion dollars of its restricted assets," U.S. Secretary of State John Kerry said.

Extension might not result in agreement

"Let me be clear," Kerry said in a statement. "Iran will not get any more money during these four months than it did during the last six months, and the vast majority of its frozen oil revenues will remain inaccessible. ... We will continue to vigorously enforce the sanctions that remain in place."

But it remains uncertain whether four more months of high-stakes talks will yield a final agreement, since the underlying differences remain significant after six rounds of meetings this year.

Western nations fear Iran's nuclear program may be aimed at developing a nuclear weapons capability. Tehran denies this.

The powers want Iran to significantly scale back its nuclear enrichment program to make sure it cannot yield nuclear bombs. Iran wants sanctions that have severely damaged its oil-dependent economy to be lifted as soon as possible.

After years of rising tension between Iran and the West and fears of a new Middle East war, last year's election of a pragmatist, Hassan Rouhani, as Iran's president led to a thaw in ties that resulted in November's diplomatic breakthrough.

But Iran's new government still insists that the country has a right to develop a nuclear energy program that includes the production of atomic fuel. The West fears that this fuel, if further processed, could also be used to make bombs.