The United States withdrew from a landmark deal, which had imposed curbs on Iran's nuclear programme in return for sanctions relief, during President Donald Trump's first term.

Tehran adhered to the 2015 deal for a year after Washington's withdrawal but then began rolling back its commitments.

Efforts to revive the pact have since faltered.

Beijing is set to host Russia and Iran -- both key diplomatic partners -- on Friday for trilateral talks on Tehran's nuclear programme.

"In the current situation, we believe that all parties should maintain calm and restraint to avoid escalating the Iran nuclear situation, or even walking towards confrontation and conflict," foreign ministry spokeswoman Mao Ning told a regular briefing.

The meeting will be attended by China's Vice Foreign Minister Ma Zhaoxu, Russian Deputy Foreign Minister Sergey Ryabkov and Iranian Deputy Foreign Minister Kazem Gharibabadi, Beijing has said.

Iran's foreign ministry has said the meeting would focus on "developments related to the nuclear issue and the lifting of sanctions".

No details about the timing of the meeting have been shared.

However, Beijing has said the talks would aim to "strengthen communication and coordination, to resume dialogue and negotiation at an early time".

"China sincerely hopes that all parties can work together, continuously increase mutual trust and dispel misgivings, and turn the momentum of restarting dialogue and negotiation into reality at an early date," Mao said.

Since returning to the White House in January, Trump has called for a new nuclear deal with Tehran while reinstating his "maximum pressure" policy of sanctions.

Iran has officially ruled out direct talks as long as sanctions remain, with President Masoud Pezeshkian vowing on Tuesday that his country "will not bow in humiliation to anyone".

Trump said last week he had sent a letter to Iran's supreme leader Ayatollah Ali Khamenei urging negotiations and warning of possible military action if Iran refused.

A quarterly report by the UN's International Atomic Energy Agency (IAEA) in February said Iran had significantly increased its stockpile of highly enriched uranium to 60 percent purity -- a short step from the 90 percent needed to make a nuclear weapon.