The killing was "a serious violation of Iran's sovereignty and security, a trampling on the aims and principles of the UN Charter and the basic norms of international relations", Beijing's foreign ministry said in a statement.

"China firmly opposes and strongly condemns this," it added, calling for an "immediate halting of military operations".

The condemnation came just after Chinese state media reported a phone call between Beijing's top diplomat Wang Yi and Russian counterpart Sergei Lavrov.

During the conversation, which state news agency Xinhua said was initiated by Lavrov, Wang said the "blatant killing of a sovereign leader and the incitement of regime change" by the United States and Israel was "unacceptable".

China is "highly concerned" that the "situation in the Middle East could be pushed into a dangerous abyss", Wang told Lavrov, according to Xinhua.

"The international community must send a definite and clear message opposing the world's regression to the law of the jungle," Wang said.

Russian President Vladimir Putin on Sunday called the killing "a cynical violation of all norms of human morality and international law".

At least one Chinese citizen has been "injured" in the strikes and "some temporary travellers are stranded", Beijing's foreign ministry said late Sunday.

The statement did not provide details on the number, physical condition or location of affected Chinese nationals in the region.

Beijing has in recent days urged its citizens in Iran to evacuate from the country "as soon as possible".

Reiterating those calls Sunday, the foreign ministry said Chinese passport holders could leave Iran on visa-free land routes into Azerbaijan, Armenia and Turkey.