Israel and the United States have been conducting strikes on Iran and Lebanon, with the conflict extending to regional neighbours.

"The most urgent task is an immediate cessation of military operations and preventing a spread and spillover of conflict," foreign ministry spokeswoman Mao Ning told a news conference, urging "a resolution through dialogue and negotiation".

One Chinese citizen was killed as a result of the conflict in Tehran, she said, where Israel and the United States have been carrying out strikes in an operation that killed the Islamic republic's supreme leader.

"The foreign ministry has instructed the Chinese embassy in Iran to provide assistance to the individual involved and the family," Mao added, without giving further details.

China was not informed in advance about the US military action, she told reporters, adding that Beijing and Moscow have urged the UN Security Council to hold an emergency meeting.

Beijing warned its citizens last week against travelling to Iran for the time being "in light of the current security situation".

More than 3,000 Chinese nationals have left Iran as of Monday.

Iran's Revolutionary Guards have vowed to launch the "most ferocious" operation in history against Israel and US bases, which are located in Gulf countries that were already counting the cost of an unprecedented series of deadly Iranian strikes.

US President Donald Trump is set to visit China from March 31 to April 2 for his first trip to the Asian power of his second term.

Asked about the planned visit, Mao said on Monday that Beijing and Washington were "maintain communication regarding the interactions between the leaders of the two countries", without providing further detail.