US Secretary of State John Kerry has cancelled a planned return to the US to stay in Switzerland as the Iran nuclear talks approach their deadline.
The State Department confirmed that Mr Kerry would be staying in Lausanne for more talks.
Foreign ministers from six world powers are trying to reach a deal with Iran to restrict its nuclear programme in return for an easing of sanctions.
They are working towards a self-imposed deadline of 31 March.
Mr Kerry had been looking forward to attending an event to honour his late Senate colleague Edward Kennedy, but had decided to stay at the ongoing talks with Iran in Switzerland, the State Department said.
At the talks in Lausanne, Mr Kerry is already joined by the German and French foreign ministers, Frank-Walter Steinmeier and Laurent Fabius, as well as his Iranian counterpart Mohammad Javad Zarif.
The German and French foreign minister are considering delaying a planned trip to Kazakhstan in order to focus on the negotiations, officials close to the talks say.
The foreign ministers of Russia, China and the UK are due to arrive on Sunday as talks go into their final three days before the end of March deadline.
At the scene: Lyse Doucet, BBC News, Lausanne
Negotiators are close, closer than ever before, in their 12-year standoff over Iran’s nuclear programme. But a deal is still not done, and no one can say with 100% percent certainty if it will be.
With each day that slips by before an end-of-March deadline for a framework agreement, the political temperature rises against the serene backdrop of the snow-capped Swiss Alps. As foreign ministers and officials stream in and out of meetings in the gilded Beau Rivage, snippets and statements to the persistent press play into 11th-hour brinkmanship.
The last difficult details are as much about political power as they are about nuclear energy. Both Iran and world powers urge the other side to make tough decisions. All say they’ve come here, hoping to make a deal, to make history.
Beyond this rarefied world, sceptics wait in many capitals, ready to react if, in their view, a bad deal emerges. Reaching what counts as a “good deal” for all will go right down to the wire and, possibly, beyond.
Israel’s Prime Minister Benjamin Netanyahu has again warned of a deal with Iran, describing it as worse than his country had feared.
On Sunday he told a cabinet meeting “this deal, as it appears to be emerging, bears out all our fears, and even more than that”.
He gave no details, but noting advances by Iran-backed forces in Yemen and other Arab countries, he accused Iran of trying to “conquer the Middle East” while pursuing nuclearisation.
“The Iran-Lausanne-Yemen axis is very dangerous to humanity and must be stopped,” he said.
Negotiators from the P5+1 group – the five permanent member of the UN Security Council plus Germany – want to ensure that Iran cannot develop nuclear weapons.
Iran denies it is aiming to build nuclear weapons and is hoping that a deal will lead to an end of international sanctions imposed on Tehran.
On Saturday, Germany’s Foreign Minister Frank-Walter Steinmeier said that after 12 years of negotiations with Iran the talks had entered their “endgame”.
Iranian Foreign Minister Mohammad Javad Zarif said that progress was being made.
After meeting Mr Steinmeier and Mr Fabius, he said: “I think we can in fact make the necessary progress to be able to resolve all the issues and start writing them down in a text that will become the final agreement.”
Mr Fabius added a note of caution, saying: “We have moved forward on certain points, but on others not enough.”
Some of the most contentious issues remain unresolved, says the BBC’s Barbara Plett Usher, in Lausanne.
Potential sticking points in the nuclear talks are thought to include the pace at which sanctions would be lifted, how long the deal will last and how much of Iran’s nuclear facilities will be open to inspection.