The Pentagon said President Donald Trump ordered Soleimani's "killing," after a pro-Iran mob this week laid siege to the US embassy in Baghdad.

Iran's supreme leader Ayatollah Ali Khamenei vowed to take "severe revenge" after Soleimani was killed when a volley of missiles hit Baghdad's international airport, striking a convoy belonging to the Hashed al-Shaabi, an Iraqi paramilitary force with close ties to Tehran.

"China has always opposed the use of force in international relations," Chinese foreign ministry spokesman Geng Shuang said at a daily press briefing.

"We urge the relevant sides, especially the United States, to remain calm and exercise restraint to avoid further escalating tensions," Geng said.

He said Iraq's sovereignty, independence and territorial integrity must be respected.

China, a permanent member of the UN Security Council, is a key partner of Iran and major buyer of the country's oil.

Geng said China urged all sides to abide by the principles of the UN charter and the "basic norms of international relations".

Iran, China and Russia held joint naval drills in the Indian Ocean and the Gulf of Oman last week and the Iranian foreign minister visited Beijing earlier this week.

China and Russia are also parties to the 2015 nuclear deal with Iran, from which Trump withdrew in May last year.