The Iran nuclear talks shifted Friday to a blame game, as Iran's foreign minister accused the United States of shifting its demands and dismissed a warning that the U.S. was ready to quit the negotiations.

Hours after his comments, Iranian Foreign Minister Mohammad Javad Zarif met again with U.S. Secretary of State John Kerry for another attempt at resolving the differences standing in the way a landmark deal that offers Iran sanctions relief in exchange for long-term, verifiable curbs on nuclear programs that Tehran could use to make weapons.

Afterward, Kerry spoke of progress, while acknowledging a "couple of very difficult issues" still blocking a deal. And despite the sharp public comments by both sides, he said the negotiating atmosphere was "very constructive."

Still, the sense of drift grew. A senior U.S. official said the preliminary April deal that set up the present negotiations had been extended until Monday. That effectively set a fourth target date for a final accord that initially was supposed to be sealed by June 30.

The tougher rhetoric Friday mirrored the frustrations by the sides as the current round of talks entered its 14th day. After blowing past two extensions, negotiators had hoped to wrap up the talks by Friday, but Zarif's comments cast doubts that agreement was near.

The sides had hoped to seal a deal before the end of Thursday in Washington to avoid delays in implementing their promises.

By missing that target, the U.S. and Iran now have to wait for a 60-day congressional review period during which President Barack Obama cannot waive sanctions on Iran. Had they reached a deal by then, the review would have been only 30 days.

Iran is unlikely to begin a substantial rollback of its nuclear program until it gets sanctions relief in return.

"If you drive the talks into a dead end then it will be you who will be committing a strategic mistake," Iran's parliamentary speaker Ali Larijani said at Friday prayers following the rally in Tehran, addressing the U.S. "And its outcome will not benefit you since Iran's nuclear staff are ready to accelerate nuclear technology at a higher speed than before."

The talks are formally between Iran and six world powers but have devolved into U.S.-Iranian negotiations over recent months, with diplomats saying the other nations were ready to accept terms agreed to by Tehran and Washington. Zarif's critical comments were thus seen as mostly directed against Washington.

Still, disagreements also have surfaced recently between the U.S. and Russia. Moscow supports Iranian demands for at least a partial lifting of the conventional arms embargo as part of any deal. That's something Washington opposes -- and an issue Zarif appeared to touch on in his comments to Iranian state television.

Beyond "witnessing a change of stances" from the other side, Zarif noted a "different stand" on some issues among the six nations. "This situation has made the work difficult," he said.

Kerry had warned Thursday the Americans were ready to leave the talks, declaring "we can't wait forever for a decision to be made." Zarif, in contrast, said his side was ready to stay and work for a "dignified and balanced deal."

Foreign ministers or top deputies of all seven nations are expected to join the talks before any impending deal. But while Germany's foreign minister remains in Vienna, the Russians and Chinese left days ago, and their French and British counterparts followed Friday, reflecting the lack of substantial advances.

British Foreign Secretary Philip Hammond spoke of "painfully slow" progress, telling reporters ministers planned to re-group Saturday "to see if we can get over the last hurdles."

The scope of access to U.N. inspectors monitoring Iran's nuclear program remains one of the sticking points in the talks. The Americans want no restrictions, whereas Iranian officials say they are concerned that unrestricted monitoring could be a cover for Western spying.

Diplomats say Iran's negotiators have signaled a willingness to compromise on the issue, but hardliners in Iran remain opposed to broad U.N. inspections. In a message directed to "negotiators on both sides," Iran's military spokesman, Gen. Masoud Jazayeri, told Iran's Fars news agency that "access to the military sites will not be allowed under any circumstance."

Anti-American sentiment remains strong in Iran, though Iranians overwhelmingly welcomed the preliminary nuclear accord in April.

On Friday, tens of thousands of Iranians taking part in an annual pro-Palestinian rally marched in Tehran, chanting "Down with America!" and "Death to Israel!"