Friday's scheduled summit in Beijing will focus on the Iranian "nuclear issue" - according to a Chinese foreign ministry statement, involving both nations' deputy foreign ministers.

China's Vice Foreign Minister Ma Zhaoxu will chair the meeting, the statement described. Iran's foreign ministry spokesperson Esmail Baqaei confirmed that topping the agenda will be "developments related to the nuclear issue and the lifting of sanctions."

The UN atomic watchdog IAEA has recently warned that Tehran "dramatically" accelerating enrichment of uranium to up to 60% purity. The Iranians are within reach of achieving weapons-grade level of roughly 90% - if they desire to pursue it.

"China has said it supports Iran in safeguarding its legitimate rights and calling for an early resumption of the Iranian nuclear talks," Reuters has reported.

Late last week President Trump said he sent a formal letter to Iran's Supreme Leader urging the restart of fresh nuclear talks, which the Iranians have scoffed at. The letter is still said to be en route via the UAE ambassador - after Iran initially said it never received such a letter.

Iranian leaders have several times since last week explained that they can't trust any dialogue or deal with Washington after it pulled out of the Joint Comprehensive Plan of Action nuclear deal in 2018 (under the first Trump administration).

Meanwhile, China, Iran and Russia are growing more cooperative on the military front, having held the "Maritime Security Belt 2025" joint naval exercises in the Gulf of Oman near the Strait of Hormuz this week.

"For two days, the crews of the warships conducted both daytime and nighttime live-fire exercises, using heavy machine guns and small arms to engage targets simulating unmanned boats and aerial drones of a mock adversary," a Russian statement said.