World powers huddled again Saturday behind closed doors on the 15th day of negotiations seeking a deal to curb Iran's nuclear ambitions, with no sign of any end to the nail-biting deadlock.

Despite a warning from U.S. Secretary of State John Kerry that he would not sit at the negotiating table forever, an Iranian official told AFP the talks, now entering their third week, could stretch on and on.

U.S. Secretary of State John Kerry said progress had been made Friday in tough talks with Iran on its nuclear program, praising what he called the "constructive" atmosphere.

"I think we have resolved some of the things that were outstanding and we’ve made some progress," he said, speaking to a few reporters as he met with his team of experts in Vienna.

Tens of thousands marched in Tehran and Baghdad Friday in annual Quds (Jerusalem) Day demonstrations in support of Palestinians, but Saudi Arabia this year joined arch-foe Israel as the target for protesters.

President Hassan Rouhani attended but did not speak at the main rally in Tehran, which coincided with seemingly deadlocked nuclear talks between Iran and world powers led by the United States.

They are among the world's most popular websites and are vital diplomatic channels for Iranian officials involved in the nuclear talks. Yet Twitter, Facebook and YouTube remain banned in Iran.

While Foreign Minister Mohammad Javad Zarif leads the way on social media -- his Twitter account has "Verified" blue tick status -- he is far from alone in seeing its benefits.

U.S. Secretary of State John Kerry said Thursday that major world powers would not be "rushed" into a nuclear deal with Iran, adding he was prepared to walk away if "tough decisions" are not made soon.

Speaking on the eve of a deadline to present a deal to the U.S. Congress, Kerry told reporters that because "the stakes are very, very high, we will not rush and we will not be rushed."

Foreign ministers from major powers sought Thursday to turn the screws on a hard-bargaining Iran to finalize a historic nuclear deal on the eve of a deadline to present it to U.S. lawmakers.

If the U.S. Congress does not receive the text by early Friday morning Vienna time -- midnight in Washington -- it makes the approval process longer and potentially more problematic.

Iranian President Hassan Rouhani expressed optimism Wednesday that a nuclear deal with world powers is imminent, saying his government is already preparing its economy for the lifting of sanctions.

"Negotiations with the P5+1 group are at a sensitive stage and the Islamic republic of Iran is preparing for (the period of) post-negotiations and post-sanctions," he said.

Iranian Foreign Minister Mohammad Javad Zarif on Wednesday said a nuclear deal with Western powers was "within reach" and could open the way to joint action against extremism, as talks continued in Vienna.

"Iran is ready to strike a fair and balanced deal and prepared to open new horizons to address the shared challenges of far greater magnitude," Zarif wrote in a commentary piece in the Financial Times.

In a game of high-stakes diplomatic brinkmanship, global powers readied Wednesday for a late-night push to forge a deal curbing Tehran's nuclear ambitions and ending a 13-year standoff.

With all bets off after ministers missed yet another deadline to seal the accord, U.S. Secretary of State John Kerry huddled with his team in a rainy Vienna to pore over documents seeking to find a way forward.

A top-level meeting at the Iran nuclear talks this week was a stormy affair, Iranian media reported Wednesday, with the country's foreign minister warning: "Never threaten an Iranian."

The altercation happened on Monday evening as foreign ministers from Iran and six major powers including U.S. Secretary of State John Kerry met in Vienna seeking to nail down a historic nuclear accord.