The negotiations, aimed at blocking Iran's capacity to build a nuclear bomb in exchange for lifting sanctions, have become bogged down over crucial details of the accord, even as the broad outlines of an agreement have been reached.

After negotiators passed an original self-imposed deadline of midnight on Tuesday, they remained locked in talks through to the early hours of Thursday in the Swiss city of Lausanne.

"It is going to be long," said a senior diplomat.

Marie Harf, spokeswoman for U.S. Secretary of State John Kerry, said on her Twitter account that negotiators had stopped at 6.00 a.m local time (0400 GMT) for a break and would resume in a few hours. Kerry and German Foreign Minister Frank-Walter Steinmeier said they would stay at least until Thursday in an effort to seal the "political" agreement, a milestone towards a final pact due by the end of June.

In a potentially hopeful sign, French Foreign Secretary Laurent Fabius returned for more talks after flying back to Paris the previous day because progress had been too slow.

"We are a few meters from the finishing line, but it's always the last meters that are the most difficult. We will try and cross them," he said on his return. "We want a robust and verifiable agreement and there are still points where there needs to be progress especially on the Iranian side."