The United States and Iran have begun a third round of nuclear talks, entering what experts describe as a more difficult phase of technical negotiations this weekend as Washington lays out its conditions.

The third round of talks in three weeks raised hopes for an Iran nuclear accord, which could avert a new Mideast conflict. President Trump said he believed a deal was within reach.

An Iranian official, briefed about the talks, told Reuters that the experts-level negotiations "are difficult, complicated and serious", without elaborating.