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's nuclear watchdog said Iran had significantly increased its stockpile of highly enriched uranium.

China's Vice Foreign Minister Ma Zhaoxu, Russian Deputy Foreign Minister Sergei Ryabkov and Iranian Deputy Foreign Minister Kazem Gharibabadi met on Friday and "exchanged views on the Iran nuclear issue and other issues of common concern", Beijing 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.

"The relevant parties should work to eliminate the root causes of the current situation and abandon sanctions, pressure, and threats of the use of force," he said.

China's top diplomat Wang Yi will meet the delegates later in the day, Ma said.

Beijing has said it hopes Friday's talks will "strengthen communication and coordination, to resume dialogue and negotiation at an early time".

- '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.

"We will enter into direct negotiations when we are on an equal footing, free from pressure and threats, and are confident that the national interests of the people will be guaranteed."

The same day, Washington upped its pressure campaign 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.

A 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 for a nuclear weapon.

Khamenei said this week that Iran "does not have nuclear weapons" and was "not seeking" them. Tehran has previously said that its nuclear programme is for peaceful purposes.