Iran and six powers agreed to continue talking for four more months after failing to meet a July 20 deadline to reach a deal on curbing the Iranian nuclear program in exchange for ending sanctions, enabling Tehran to access $2.8 billion of frozen cash, Reuters reported.

But U.S. officials warned that most sanctions against the Islamic Republic would remain in place.

The announcement came in the early hours of Saturday, July 19, after nearly three weeks of marathon talks in a 19th century Viennese palace, where senior officials from Iran, the United States, Britain, France, Germany, Russia and China were holed up in negotiating rooms struggling to reach an agreement.

Iran will be allowed to access in tranches an additional $2.8 billion of its frozen assets during the period of extended talks, senior U.S. officials told reporters in Vienna.

"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," U.S. Secretary of State John Kerry in a statement released in Vienna on Saturday. "We will continue to vigorously enforce the sanctions that remain in place."

It remains uncertain whether four more months of high-stakes talks will yield a final deal, since major underlying differences remain after six rounds of meetings this year, Reuters says.

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

In exchange for the $2.8 billion, Kerry said, Iran has agreed to continue neutralizing its most sensitive uranium stocks - uranium that has been enriched to a level of 20 percent purity - by converting it to fuel for a research reactor in Tehran that is used to make medical isotopes.

Kerry said the future of Iran's enrichment program was one of the most divisive topics.

"There are very real gaps on issues such as enrichment capacity at the Natanz enrichment facility," he said. "This issue is an absolutely critical component of any potential comprehensive agreement. We have much more work to do in this area, and in others as well."

Iranian Foreign Minister Mohammad Javad Zarif told reporters earlier this week that Tehran would be willing to delay development of an industrial-scale uranium enrichment program for up to seven years and to keep the 19,000 centrifuges it has installed so far for this purpose.

But Kerry said after several face-to-face meetings with Zarif it was "crystal clear" that for Iran to keep all of its existing centrifuges was out of the question.