At least two people sustained minor injuries when 14 rockets were fired at the joint US-Iraqi Ain Al Asad airbase on Wednesday.
The missiles landed on the base and the perimeter at about 12.30pm, US coalition spokesman Wayne Marotto said.
Ain Al Asad, in Anbar province, hosts the largest contingent of US troops in Iraq and a small number of other international forces. Most troops on the base are Iraqi.
US forces activated defensive measures as the stream of rockets was fired. Some hit the base, while others struck the outer boundary, Col Marotto said.
Iraqi security forces said the origin of the attack was a small lorry disguised as a civilian vehicle carrying flour. It had been modified to carry a crude multiple-launch rocket system.
A mosque and some buildings were damaged near the site of the rocket launch, the Coalition said.
An assault was also mounted against a US base in Syria.
An explosive drone was launched at American and allied Kurdish forces at Al Omar oilfield in Deir Ezzor, local media reported. It was apparently intercepted.
The attacks followed drone attacks against the US embassy in Baghdad and the US consulate in Erbil, northern Iraq, on Tuesday.
An improvised drone in Baghdad which carried a small munition was reportedly shot down. The Erbil attack also caused no casualties.
The US embassy activated its Counter Rocket, Artillery and Mortar system defences, sending streams of orange tracer bullets arcing across the night sky.
Iraqi Ministry of Defence spokesman Yehia Rasool condemned the attacks as acts of terrorism “targeting the sovereignty and security of our country”.
Conflict between the US and Iran-backed militia groups has escalated in recent months. Washington has blamed them for attacks against American troops in Iraq and Syria.
US and Iran-backed militia stalemate
Since US President Joe Biden’s inauguration on January 22, Iran-backed groups in Iraq have launched close to 30 attacks on US forces, according to analysis by Michael Knights of the Washington Institute for Near East Policy, a think tank.
US and international forces in Iraq are training the Iraqi army to stymie an attempted revival of ISIS. But Coalition forces have increasingly come under attack by Iran-backed militias, part of a government-sanctioned force known as the Popular Mobilisation Units.
Under President Trump, Washington made clear the red line for the militias was the death of an American citizen.
That line was crossed with the death of an Iraqi-American contractor at a base near Kirkuk on December 27, 2019. In response, the US launched heavy air strikes against Iran’s main proxy militia force in Iraq, Kataib Hezbollah.
The resulting escalation cycle brought Iran and the US to the brink of war on January 3, 2020, when a US air strike in Baghdad killed Iranian general Qassem Suleimani and the head of Kataib Hezbollah, Abu Mahdi Al Muhandis.
Mr Biden has changed tack since taking office. On the one hand, he has established a deterrence policy whereby even non-lethal attacks will be met with US air strikes.
But unlike his predecessor Donald Trump, Mr Biden is aiming for proportionate counter strikes rather than heavy air strikes that could kill dozens of militiamen and worsen the conflict.
Calculated proportionality also risks inviting more attacks, says Alex Almeida, a security analyst at Horizon Client Access, who focuses on non-state armed groups.
“The militias are threading the line between keeping the revenge narrative going and being careful not to draw another massive Trump-style retaliatory air strike,” he said.
“It’s clear Biden is less willing to retaliate, so the bar for what they can get away with is a lot higher than under Trump.”
The Iran-backed groups have rapidly growing arsenals of drones, rockets and even short range ballistic missiles.
But the militias are falling back on smaller attacks out of caution, Mr Almeida said.
On June 28, the US bombed what it said were buildings storing explosive drones in Albu Kamal, a small town in Syria. It is near the Iraqi border town of Al Qaim, where Iran-backed militia groups have a heavy presence.
Five members of a PMF group were killed.
Those air strikes were retaliation for a non-lethal drone attack on June 26 against the US consulate in Erbil in the semi-autonomous Kurdish region.
Abu Alaa Al Walae, the head of Kataib Sayyid Al Shuhada, told AFP on Tuesday his forces were planning revenge against US forces for the attack.
“We want it to be an operation in which everyone says they have taken revenge on the Americans,” Al Walae said. “It will be a qualitative operation [that could come] from the air, the sea, along Iraq’s border, in the region or anywhere. It’s an open war.”
The violence has raised questions as to how serious the current round of fighting could become.
“I see this trending toward continued escalation but not all out escalation,” Mr Almeida said.
“We’re not likely to see a ramped roadside bombing campaign, which could seriously disrupted Coalition logistics. In other words, multiple convoy roadside bombs per week. We probably won’t see a sustained high volume of rocket fire at multiple Coalition operating locations, like in 2011,” he said.
“In general, I think the militias will continue trying out new modes of attacks, drones, anti-air missiles, maybe. But I think the overall volume of attacks is going to remain reasonably low.”
Kataib Hezbollah attacks on US forces in 2011, using much larger rockets than those typically used in recent attacks, killed 15 US servicemen in the space of a month before stopping after a combination of US threats and diplomacy.
But even if the militias and the US refrain from attempted mass casualty attacks against each other, the ongoing fight against ISIS, known by the Coalition as Operation Inherent Resolve, could be disrupted, said Joel Wing, an analyst in California who has tracked fighting in Iraq since 2008.
“I think OIR is in a tailspin. The US is going into force protection, which means it’s not doing much advising and assistance to the Iraqis. The pro-Iran camp has the initiative because it can up the ante with more intense attacks any time it wants,” he told The National.
“US deterrence has been ineffective hitting camps out on the Iraq-Syrian border, so I’ve gotten the impression that US military doesn’t like escalation.”