Beijing, China - China has strongly condemned President Donald Trump's war on Iran, accusing the US and Israel of violating international law.

"The US-Israeli strikes have no UN Security Council authorization and violate international law," said Chinese Foreign Ministry Spokesperson Mao Ning during her regular press conference on Monday.

"China is deeply concerned over the regional spillover. China believes that the sovereignty, security, and territorial integrity of the Gulf states should likewise be fully respected," she said.

"China stands ready to work with the international community to call for peace and stop the conflict, resolve issues through dialogue and negotiations, and maintain peace and stability in the Middle East and the world at large."

Mao's comments come two days after Trump, with military support from Israel, launched a massive attack on Iran, bombing much of the country and ultimately killing Ayatollah Ali Khamenei.

A Chinese citizen was also confirmed to have been killed during US strikes, which have continued throughout Monday.

"China mourns for our fellow citizen who lost his life when caught in the military conflict in Tehran," Mao said when asked about the death.

The war has also drawn into question whether Trump's planned trip to Beijing in April will go ahead, but Mao said that she has "no information to share."

"The attack and killing of Iran’s supreme leader is a grave violation of Iran’s sovereignty and security," Mao said. "China firmly opposes and strongly condemns it."

Though Western allies of the US have generally supported the assault on Iran, Spain's Foreign Minister José Manuel Albares broke ranks and condemned the strikes as "unjustified."

Brazil's government issued a statement on Saturday also expressing "grave concerns" about the attacks and urging peace.

"Brazil calls on all parties to respect international law and to exercise maximum restraint to prevent an escalation of hostilities and to ensure the protection of civilians and civilian infrastructure," the statement read.

UN Human Rights Chief Volker Türk called for restraint and said that it is "paramount" that civilians are protected under international law.

"I deplore the military strikes across Iran this morning by Israel and the United States of America, and the subsequent retaliatory strikes by Iran," he said.

Voices within the US have also risen to condemn Trump's actions, with New York Mayor Zohran Mamdani labeling it "an illegal war of aggression."

"Bombing cities. Killing civilians. Opening a new theater of war. Americans do not want this. They do not want another war in pursuit of regime change. They want relief from the affordability crisis. They want peace," Mamdani said.