LAUSANNE, Switzerland - The latest round of talks over Iran's nuclear future ended abruptly Friday with negotiators planning to resume discussions next week in a last-ditch effort to forge an agreement by a March 31 deadline.

The immediate explanation for the ending was that the Iranians had to return to Tehran for the funeral of Sekineh Payvandi, 90, the mother of President Hassan Rouhani and his brother, Hossein Fereydoun, who is a member of the team negotiating Iran's nuclear program. Word of the death reached negotiators Friday morning just before negotiations were to take place.

Despite some progress over four days of talks, many fundamental issues remain unresolved.

The United States, Europeans, and the Iranians held some abbreviated discussions Friday, their fifth day of trying to reach a framework agreement to constrain Iran's nuclear program and ease sanctions.

Outside the negotiating room there was a traditional Nowruz table laden with sweets, stuffed animals, and a pot of grass. The message seemed to be that even an important holiday like Nowruz would not deter the negotiators from their task.

President Obama and Secretary of State John Kerry urged Iran's leaders to strive for an agreement in the spirit of Nowruz, the Persian new year.

Obama, in his annual videotaped Nowruz message, said this opportunity may not come again soon. If Iran's leaders do not agree to a reasonable deal, he said, the country will remain isolated and sanctions will further squeeze its economy, which has suffered a double blow from falling oil prices.

Obama also called Friday for the release of four Americans detained or missing in Iran, arguing that the start of Nowruz offers an opportunity for the Iranian government to reunite them with their families.

Invoking the holiday, Obama asked Iranian leaders to free Christian pastor Saeed Abedini, former U.S. Marine Amir Hekmati, and Washington Post reporter Jason Rezaian, as well as locate missing American investigator Robert Levinson.

"The spirit of family is deeply woven into all of the rich cultural traditions of the Nowruz holiday," the president said in a statement. "It is a time for reuniting and rejoicing with loved ones and sharing hopes for the new year. Today, as families across the world gather to mark this holiday, we remember those American families who are enduring painful separations from their loved ones who are imprisoned or went missing in Iran."

Now, Kerry plans to return to Washington on Saturday, stopping in London to consult with European allies in the talks. He said the nuclear talks would resume Thursday.