Iraqi people take part in a protest against the announced U.S. Middle East peace plan in Baghdad, Iraq, on Jan. 31, 2020. (Xinhua/Khalil Dawood)

Top Iraqi politicians joined the United Nations Thursday in condemning a rocket attack north of Baghdad that killed a British soldier and two Americans and threatened a new escalation.

A volley of 18 rockets slammed into the Taji air base late Wednesday, killing a British soldier, a US soldier and an American contractor in the deadliest attack in years on US forces in Iraq.

There was no claim of responsibility but Washington has accused Iran-backed factions of Iraq's Hashed al-Shaabi paramilitary alliance of carrying out similar attacks.

Within hours, air raids killed 26 Iran-aligned Iraqi fighters in neighboring Syria, prompting fears that tensions between Washington and Tehran would once again flare up. 

On Thursday morning, Iraq's military command said the attack was "a serious security challenge" and pledged to open an investigation. 

President Barham Saleh and parliament speaker Mohammed al-Halbussi condemned a "terrorist attack" which targeted "Iraq and its security." 

The UN mission in Iraq called for "maximum restraint on all sides."

"These ongoing attacks are a clear and substantial threat to the country, and the risk of rogue action by armed groups remains a constant concern," it said in a statement.

The attack was the 22nd since October on US interests in Iraq. US diplomatic offices have come under attack as well as the bases where the 5,200 American troops stationed in Iraq are based.

The previous attacks killed an Iraqi soldier and a US contractor, leading to a major uptick in tensions between Washington and Tehran.

Washington responded to the American contractor's death with air strikes that killed more than two dozen Iran-backed Iraqi fighters. 

Days later, a US drone killed senior Iranian commander Qasem Soleimani and Hashed deputy chief Abu Mahdi al-Muhandis near Baghdad airport prompting retaliatory Iranian air strikes against coalition troops in Iraq. 

AFP