After meeting Iranian Foreign Minister Mohammad Javad Zarif in Brussels, Borrell tweeted: "Spoke w Iranian FM @JZarif about recent developments. Underlined need for de-escalation of tensions, to exercise restraint & avoid further escalation".

US President Donald Trump who ordered the precision drone strike in which Iran's Major General Qasem Soleimani was killed on Friday has said the military mastermind was planning an "imminent" attack on US diplomats and the roughly 5,200 American troops deployed in Baghdad.

Borrell said he also urged Zarif to maintain the landmark nuclear accord negotiated between Iran and the UN Security Council permanent members -- Britain, France, China, Russia and the United States -- plus Germany.

The deal, also known as the JCPOA, offered Tehran relief from stinging sanctions in return for curbs to prevent it acquiring nuclear weapons.

Agreed in 2015 it has been at risk of falling apart since Trump unilaterally withdrew from it in May 2018 and reimposed sanctions on Iran.

"Also discussed importance of preserving #JCPOA, which remains crucial for global security. I am committed to role as coordinator," Borrell said.

A furious Iran has vowed revenge for the killing of Soleimani, the chief architect of its military operations across the Middle East.