Iran, the United States, Britain, France, Germany, Russia and China reached a tentative framework for a nuclear pact on April 2 in Lausanne but several issues remain unresolved.

They have a self-imposed June 30 deadline to arrive at a comprehensive agreement.

Chinese Foreign Minister Wang Yi, who is in Russia, said the situation had become more complex now that the talks process was in its final stages but that everyone should be patient.

"(We) must push forward the next stage of talks on the basis of the Lausanne framework ... and all parties should not raise any new demands to prevent complicating the talks process," the foreign ministry quoted Wang as telling his Russian and Iranian counterparts.

"All sides' legitimate concerns ought to be paid attention to and rationally resolved; all sides should meet each other half way and not drift further apart," Wang added.

A swift agreement would benefit the global non-proliferation system as well as regional peace and stability, and China would continue to play a constructive role, he said.

China and Iran have close diplomatic, economic, trade and energy ties.