Iraqi and UN officials scrambled Thursday (March 12th) to contain the fallout from an unprecedented rocket attack that killed three members of the international coalition and threatened another escalation of Iran-US tensions.

Within hours of the attack on the Taji airbase north of Baghdad -- the deadliest in years on a base used by US forces in Iraq -- an airstrike killed more than two dozen Iran-aligned militiamen near the border town of Albu Kamal in Syria.

It marked a dramatic uptick in violence less than three months after rockets killed a US contractor in northern Iraq, unleashing a round of tit-for-tat attacks between Washington and Tehran on Iraqi soil.

Fearing an even bloodier flare-up this time, Iraqi officials and the UN were quick to condemn the deaths.

Iraq's military command said it was "a serious security challenge" and pledged to open an investigation.

President Barham Saleh and parliament speaker Mohammed al-Halbousi 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. "The last thing Iraq needs is to serve as an arena for vendettas and external battles."

US Secretary of State Mike Pompeo and British Foreign Minister Dominic Raab condemned the attack in a telephone call.

The two "underscored that those responsible for the attacks must be held accountable", the US State Department said in a statement.

Briton, US citizens killed

Wednesday's attack was the 22nd on US interests in Iraq since late October.

It saw a volley of 18 rockets slam into the Taji airbase, one of about a dozen facilities across Iraq where international coalition forces are based.

The Iraqi military said the rockets were fired from the back of a truck.

The coalition confirmed three of its personnel were killed and around a dozen more wounded.

One of the dead was a member of the Royal Army Medical Corps, Britain confirmed. A US military official said the other two were a US soldier and a US contractor.

There was no immediate word on Iraqi casualties.

No group claimed responsibility, but the US has accused Iran-aligned factions of Iraq's Popular Mobilisation Forces (PMF) of carrying out similar attacks.

Iran-aligned Iraqi militia Kataib Hizbullah hailed the attack and its perpetrators, without saying they were behind it.

Previous attacks linked to Iran

In late December, the US accused Kataib Hizbullah of killing a US contractor at a base in northern Iraq.

It responded with airstrikes in western Iraq that killed 25 of the group's fighters.

Days later, Islamic Revolutionary Guard Corps Quds Force commander Qassem Soleimani and PMF deputy chief Abu Mahdi al-Muhandis were killed in a US strike near Baghdad airport.

Iran then launched its own strikes on a western Iraqi base, leaving dozens of US troops suffering from brain trauma.

Iran-aligned Iraqi militias operating under the auspices of the PMF have repeatedly pledged to avenge al-Muhandis's death in their own way.

Hours after the Wednesday attack on Taji airbase, an airstrike killed 26 Iran-aligned Iraqi fighters in Syria, the Syrian Observatory for Human Rights said.

The strike, conducted in the Albu Kamal area near the Iraq border, was carried out by three planes which probably belonged to the international coalition, it said.

But the coalition denied having carried out any raids in Syria or Iraq on Wednesday night.

"The US/Coalition did not conduct any strikes in Syria or Iraq last night," a spokesman said in a statement to AFP.

Some 5,200 US troops are stationed in Iraq as part of the international coalition formed in 2014 to fight the "Islamic State of Iraq and Syria" (ISIS).

On Sunday, two US soldiers were killed north of Baghdad while helping Iraqi forces battle ISIS remnants.