The United States withdrew from a landmark 2015 deal, which imposed curbs on Tehran's nuclear development in return for sanctions relief, during US President Donald Trump's first term.

The Republican president has called for a new nuclear deal with Iran since his return to the White House in January, but Tehran says no such agreement is possible so long as punishing sanctions remain in place.

The effort to secure a pact was given new urgency last month when the UN nuclear watchdog said Iran had significantly increased its stockpile of highly enriched uranium.

Beijing hosted Iranian Deputy Foreign Minister Kazem Gharibabadi and Russian Deputy Foreign Minister Sergei Ryabkov for talks on Friday it said it hoped would help "resume dialogue and negotiation at an early time".

Meeting the diplomats, Chinese Foreign Minister Wang Yi stressed "the comprehensive agreement on the Iranian nuclear issue is an important achievement achieved through dialogue and negotiation".

"Now the situation has reached a critical juncture again. We must buy time for peace, resolve disputes through political and diplomatic means, and oppose the use of force and illegal sanctions," he said.

And the United States should demonstrate "political sincerity and return to talks at an early date", said a statement from the Chinese foreign ministry, which outlined proposals for resolving the Iran nuclear issue.

"All parties should... refrain from actions that might escalate the situation," the statement added.

- 'Peaceful' nuclear programme -

Gharibabadi, in turn, hailed the "constructive" talks in Beijing and said Iran's nuclear programme "is peaceful in nature".

"But unfortunately, some countries are trying to create an unnecessary crisis in this regard," he said.

"The main root cause of the current situation is the unilateral withdrawal of the United States."

Gharibabadi told Iran's Mehr news agency that China and Russia had backed Iran's position that any new nuclear talks should "focus solely on the nuclear issue".

One of the reasons cited by Trump for abandoning the 2015 deal was that it did not seek to address Iran's ballistic missile programme which he argued posed a threat to the United States.

The Russian and Iranian officials met China's Vice Foreign Minister Ma Zhaoxu earlier on Friday and "exchanged views on the Iran nuclear issue and other issues of common concern", Chinese state media said.

Ma reiterated after that meeting China's support for "political and diplomatic engagement and dialogue based on mutual respect".

"We emphasised the necessity of ending all illegal, unilateral sanctions," Ma told reporters.

Iran's ambassador to China said later on social media platform X the meeting was a "complete success".

The talks resulted in agreements on "trilateral cooperation on important international issues, including the need for the three countries to cooperate in confronting" US sanctions, he said.

The Kremlin's spokesman Dmitry Peskov said that "it is necessary to further continue diplomatic efforts" on Iran's nuclear programme, and condemned "illegal" sanctions against Tehran.

- 'Maximum pressure' -

Trump has reinstated his "maximum pressure" policy of sanctions against Iran, mirroring his approach during his first term.

He sent a letter to Tehran this week urging nuclear talks and warning of possible military action if it refuses.

Tehran said the letter, which Trump said was addressed to Iran's supreme leader Ayatollah Ali Khamenei, was currently "being reviewed".

"Ultimately, the United States should lift the sanctions," Foreign Minister Abbas Araghchi said in an interview published by the government's official newspaper on Thursday.

Washington upped its pressure campaign the same day by imposing sanctions on Iranian Petroleum Minister Mohsen Paknejad.

The US Treasury Department also blacklisted the owners or operators of vessels it said were engaged in transporting Iranian oil to China.