US Defense Secretary Mark Esper warned Thursday that “all options are on the table” after three US-led coalition members, including two American soldiers, were killed in a rocket attack in Iraq.

Iraqi and UN officials were scrambling meanwhile to contain the fallout from the strike on Taji air base, which threatened yet another escalation of Iran-US tensions.

Within hours of the attack on the base north of Baghdad—the deadliest in years on a facility used by US forces in Iraq—an air strike killed more than two dozen Iran-aligned fighters in neighboring 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 the United States and Iran on Iraqi soil.

In Washington, Esper blamed the attack on Iranian-backed Shia militia groups and told reporters President Donald Trump had given him “the authority to do what we need to do.”

“We have pretty good confidence we know who did this,” he said, insisting they were “clearly targeting coalition and partner forces on Camp Taji.”

“Let me be clear, the United States will not tolerate attacks against our people, our interests or our allies,” the Pentagon chief said.

“All options are on the table as we work with our partners to bring the perpetrators to justice and maintain deterrence,” Esper said. “As we demonstrated in recent months, we will take any action necessary to protect our forces in Iraq and the region.”

General Mark Milley, the chairman of the Joint Chiefs of Staff, said 30 rockets were fired, 18 of which hit the base.

Two US servicemen and one British soldier were killed while another 14 coalition members were injured, five of whom were in serious condition, Milley said.

‘Serious security challenge’

Fearing a bloody flare-up, Iraqi officials and the United Nations were quick to condemn the coalition 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-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. “The last thing Iraq needs is to serve as an arena for vendettas and external battles.”

Wednesday’s attack on the Taji base, one of about a dozen facilities across Iraq where coalition forces are posted, was the 22nd on US interests in Iraq since late October.

Kataeb Hezbollah, a hardline faction within Iraq’s Hashed al-Shaabi paramilitary alliance, hailed the attack and its perpetrators, without saying they were behind it.

“We believe it is the best time for popular, nationalist forces to resume operations to oust the evil attackers,” the group said in a statement.

Kataeb Hezbollah also criticized “those who were quick to denounce and express their sympathy,” in a hint at top Iraqi officials who had condemned the rocket attack.

In late December, the United States accused Kataeb Hezbollah of killing an American contractor at a base in northern Iraq and carried out air strikes on western Iraq that killed 25 of its fighters.

Days later, a US drone strike killed senior Iranian commander Qasem Soleimani and Hashed deputy chief Abu Mahdi al-Muhandis near Baghdad airport.

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

Hashed factions have repeatedly pledged to avenge Muhandis’s death in their own way.

Hashed hammered in Syria

Within hours of Wednesday’s attack, an air strike near the Syrian-Iraqi border town of Albu Kamal killed 26 Iran-aligned Iraqi fighters, the Syrian Observatory for Human Rights said.

The US-led coalition denied carrying out any raids overnight on either Syria or Iraq.

Both the coalition and Israel have targeted Iran-backed fighters in Syria, whom they fear could be transferring missiles from their regional foe Iran.

Post-Saddam Iraq counts years of close ties with both Iran and the United States, and Baghdad has been put in an increasingly difficult position by the spiralling tensions between its allies.

In January, Iraqi lawmakers voted to oust all foreign troops from Iraq in reaction to the killing of Soleimani and Muhandis.

Some 5,200 US troops are stationed in Iraq as part of the coalition formed in 2014 to fight the Islamic State jihadist group.

While IS has lost all the territory it once held in Iraq and Syria, sleeper cells remain capable of carrying out attacks on both sides of the border.

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

US officials have previously told AFP they consider the Hashed a bigger threat than IS, given the frequency and accuracy of rocket attacks on US troops that could be traced back to the paramilitaries.