Chinese Foreign Minister Wang Yi has returned to Vienna for the second time in a week to join the Iran nuclear talks as a self-imposed deadline for a deal is due to expire today.
Wang says the talks have achieved significant progress but differences remain.
"There are still a few issues left on the conference table, and from our point of view, it is possible that we could find solutions for these issues. That is to say, a comprehensive Iran nuclear agreement is at our fingertips. Monday and Tuesday will be the critical time for all parties, especially the United States and Iran, to make a final political decision."
As part of Monday's session, Wang Yi had a meeting with Russian Foreign Minister Sergey Lavrov.
Iran and the five UN Security Council permanent members plus Germany are trying to reach a final comprehensive agreement on Iran's controversial nuclear program.
Both parties are hoping to clinch a deal today, setting a decade of restrictions on the nuclear program and granting Iran significant relief from international sanctions.
Some Western officials say a dispute over U.N. sanctions on Iran's ballistic missile program and a broader arms embargo are among issues holding up a deal.
Iran is reportedly insisting that all the UN sanctions, including on the ballistic missiles, should be lifted but world powers want it to be remain under sanctions.
Separately, a senior Iranian official says Tehran wants a UN arms embargo terminated as well but a Western diplomat says a removal is "out of the question".
