A deal to deny Iran an arsenal of nuclear warheads remained unfinished as the world's big powers and Tehran failed again to meet a self-imposed deadline, only to set a new one for Monday.
Talks will resume Saturday in Vienna, but stark differences on key issues – intrusive inspections of Iran's military bases, whether ending sanctions also means lifting the arms embargo – may still derail a deal.
After weeks of intense diplomacy failed to meet a twice-extended deadline, both sides resorted to veiled threats and traded accusations.
Iranian Foreign Minister Mohammad Javad Zarif accused the United States and its Western allies of demanding additional, last-minute requirements. "Now they have excessive demands," he said.
President Barack Obama still hopes to announce what would be an historic breakthrough and provide a significant foreign-policy achievement to his legacy.
Iran and the U.S. have been bitter adversaries since 1979, when the Islamic Revolution toppled the pro-Western Shah and radicals seized the U.S. embassy, holding 52 diplomats and citizens hostage for 444 days.
Mr. Obama has sought better relations with Tehran throughout his presidency, and a nuclear pact could lead to normalization of relations between the U.S. and a major Middle East power.
"The President's schedule has been left intentionally fluid to account for the fact that we may have news out of Vienna," said White House spokesman Josh Earnest.
But Secretary of State John Kerry, who has spent weeks in Vienna trying to close the deal, was also warning that time was running out.
"This is not open-ended," Mr. Kerry said, telling Tehran that "if the tough decisions don't get made, we are absolutely prepared to call an end to this process."
For Mr. Kerry, still on crutches after breaking his hip in a biking accident, the tough talks in Vienna mark the longest continuous overseas effort by a U.S. secretary of state on a single issue since Henry Kissinger's 34-day shuttle diplomacy seeking Middle East peace in 1974 after the Arab-Israeli war.
"Some of the tough issues remain unresolved," Mr. Kerry admitted Friday as another deadline loomed. "We shouldn't get up and leave just because the clocks strikes midnight. We will not rush, and we will not be rushed."
Mr. Kerry said any pact must stand the test of time, lasting for decades, but draft versions suggest Tehran will only be constrained for 10 years.
"If, in fact, Iran is willing to agree to double-digit years of keeping their program where it is right now and, in fact, rolling back elements of it that currently exist … if we've got that, and we've got a way of verifying that, there's no other steps we can take that would give us such assurance that they don't have a nuclear weapon," the President said in March.
Earlier this week, Mr. Obama told Democrats that he put the chances of success in reaching a nuclear limitation deal with Tehran at "less than 50-50."
The major Western powers suspect Iran's vast, clandestine and often buried nuclear enrichment and technology programs are intended to secretly develop nuclear warheads for the nation's already sophisticated ballistic missile program. Tehran insists its nuclear program, including thousands of enrichment centrifuges, is entirely peaceful and intended only to develop power generation and medical isotopes.
Britain, China, France, Russia, the U.S. and Germany – the so-called P5+1 group, referring to the five permanent members of the Security Council plus Germany – have been meeting with Iran for months in on-again, off-again talks aimed at resolving the dispute.
In exchange for any nuclear pact, Iran, crippled by sanctions, wants immediate relief and a path out of the isolation imposed on it by decades of U.S.-led international pressure.
Meanwhile in Tehran, tens of thousands demonstrated after prayers on al-Quds Day, a holiday designed by Ayatollah Ruhollah Khomeini to show support for Palestinians on the last Friday of the month of Ramadan. Chanting "Down with America," protesters burned American and Israeli flags as well as effigies of Mr. Obama and Israeli Prime Minister Benjamin Netanyahu.