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"The secretary-general has consistently advocated for de-escalation in the Gulf," a spokesman for Guterres said in a statement.

"This is a moment in which leaders must exercise maximum restraint. The world cannot afford another war in the Gulf."

The United States announced early on Friday that it had killed Qasem Soleimani, the commander of the Iranian Quds Force, in a precision drone strike on Baghdad's international airport.

Read more: Who was Qasem Soleimani, the powerful commander of Iran's elite Quds force?

Iran has declared it will seek to avenge the killing of Soleimani, widely considered among the most powerful figures in the Islamic Republic, second only to the supreme leader Ayatollah Khamenei.

Despite the fierce war of words between Iran and the US following the attack, the worrying escalation prompted other world leaders and opponents of Trump to issue pleas for restraint.

"President Trump just tossed a stick of dynamite into a tinderbox," former vice president Joe Biden said in a statement.

"Iran will surely respond. We could be on the brink of a major conflict across the Middle East."

The UN is on the brink of re-issuing sanctions on Iran, as Tehran pushes ahead with uranium enrichment, in breach of the limits on its nuclear programme imposed by the 2015 nuclear deal.

President Trump unilaterally withdrew from the deal brokered by his predecessor Barack Obama, reimposing harsh sanctions on Iran and sparking renewed tensions which have mounted until now.

Agencies contributed to this report.