Iran and six world powers have given themselves until July 7 to reach a comprehensive nuclear agreement.
Iran, Britain, China, France, Germany, Russia, and the United States made the decision on June 30 as a midnight deadline approached in talks in Vienna without a deal.
Negotiators are working to strike an agreement under which Tehran would curb its nuclear program in exchange for sanctions relief.
The negotiations involved U.S. Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif, their Russian counterpart Sergei Lavrov, and others.
"There remain questions, mostly regarding procedural issues rather than technical,” Lavrov said after meeting Kerry. “We have all reason to believe that results are within reach."
At the White House, President Barack Obama reiterated he would "walk away" from a deal with Iran if the conditions are not satisfactory.
Iranian President Hassan Rohani said many sticking points had been resolved but some remained.
"Some of these can be sorted out in the coming days if there are no excessive demands," he added.
The six powers want limits on Tehran's nuclear programs that could have a military use.
Tehran denies it is pursuing atomic weapons.