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. 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.