Hours away from a Monday deadline for completing a new accord to curb Iran’s nuclear program, negotiators planned to extend negotiations and expected to reconvene next month, a Western diplomat said.

A location for the December talks has yet to be chosen, but over the past month, Secretary of State John Kerry has met with his Iranian counterpart, Mohammad Javad Zarif, in Muscat, Oman, and in Vienna.

American officials and their negotiating partners have yet to explain if any substantial progress was made in the latest round of talks here and what gaps remain. President Obama said in a television interview on Sunday that there were still “significant” differences between the two sides.

It was also unclear how long the talks would be prolonged, as negotiators try to resolve crucial issues, including how much nuclear fuel Iran could produce, how long the accord would last and how intrusive inspections would be.