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By Andrea Mitchell

Negotiators from six world powers are making encouraging progress with Iran on a deal over curtailing its nuclear program, but it remains unclear whether they will hit Monday's self-imposed deadline, officials told NBC News.

Several sticking points remain, including Iran's demand — supported by Russia — that a U.N. embargo on conventional arms, including ballistic missiles, be dropped as part of an agreement.

But with top diplomats from Russia and China reportedly on their way to Vienna to rejoin the talks Sunday, the final pieces may come together overnight, subject to approval to the governments of all seven countries involved.

Related: Kerry: Nuclear Deal Must 'Stand the Test of Time'

A senior U.S. State Department official remained cautious, telling NBC News that an overnight deal was possible, but that "major issues remain to be resolved."

Any report that a deal is imminent is overly optimistic, U.S. officials said.

Iranian officials struck a similarly cautious note.

Iran and six powers — Britain, China, France, Germany, Russia and the United States — are attempting to end a more than 12-year dispute over Iran's atomic program by negotiating limits on its nuclear activities in exchange for sanctions relief.

Negotiators have until Monday afternoon to secure a deal, after the deadline was extended on Friday.

U.S. Secretary of State John Kerry told reporters in the Austrian capital Sunday morning that he was "hopeful."

Iranian President Hassan Rouhani added, "It might seem we have reached the top of the mountain. But no, there are still steps needed to be taken,” the state news agency ISNA reported, according to Reuters.