BAGHDAD - Iraqi and United Nations officials scrambled Thursday to contain the fallout from an unprecedented rocket attack that killed three US-led coalition members and threatened yet another escalation of Iran-US tensions.
Within hours of the attack on Taji air base, north of Baghdad - the deadliest in years on a base used by US forces in Iraq - an air strike killed more than two dozen Iran-aligned fighters in neighbouring Syria. Syria’s state media reported late Wednesday that unidentified jets hit targets south east of the Syrian town of Albu Kamal along a strategic border crossing with Iraq.
The attacks 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 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."
US Marine General Kenneth McKenzie, the head of the US military's Central Command, did not blame any specific militia for the attack in Iraq but noted that only the Iran-backed Kataeb Hezbollah had been known to wage such an attack in the past.
"While we are still investigating the attack, I will note that the Iranian proxy group Kataeb Hezbollah is the only group known to have previously conducted an indirect fire attack of this scale against US and coalition forces in Iraq," McKenzie told a US Senate hearing.
Kataeb Hezbollah hails attack
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 base, one of about a dozen facilities across Iraq where coalition forces are posted.
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 an American contractor.
There was no immediate word on Iraqi casualties and no group claimed responsibility.
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 criticised "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 US 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 on the Euphrates River killed 26 Iran-aligned Iraqi fighters, the Syrian Observatory for Human Rights said.
Iran-backed Iraqi Shiite militias known as the Popular Mobilisation Forces (PMF) captured Albu Kamal from the Islamic State toward the end of 2017. The border town lies on a strategic supply route for Iranian-backed militias who regularly send reinforcements from Iraq into Syria to aid Syrian President Bashar al-Assad's forces.
Control of the border crossing is crucial for Iran’s bid to cement its growing sway over a corridor of territory from Tehran to Beirut. Iranian-backed militias are also in control of large stretches of the frontier on the Iraqi side.
The US-led coalition, however, denied carrying out any raids overnight on either Syria or Iraq. Both the coalition and Israel have previously targeted Iran-backed fighters in Syria, accusing them of transferring missiles from Iran.
The Hashed also blamed Israel and the US for a string of unexplained explosions last year.
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 of 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 said they considered 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.