The rocket fire the previous day was the latest in a series of attacks targeting Ain al-Assad base in recent weeks, which hosts American troops as well as other personnel from the US-led coalition against the Islamic State jihadist group.

It came at a time of already heightened tensions in the Middle East, with the region awaiting an expected counterattack by Iran on Israel.

"Five US service members and two US contractors were injured in the attack," in which two rockets struck the base, a US defence official said on condition of anonymity.

Five of the wounded are being treated at the base while two have been evacuated for further treatment, the official said, adding that all are in stable condition.

US Defence Secretary Lloyd Austin later told journalists that "the United States will not tolerate attacks on our personnel in the region."

"In terms of who was behind the attack on Al-Assad, we're sure that it was an Iranian-backed Shiite militia group," but the investigation into which one is still ongoing, Austin said.

The Iraqi government's security media unit said that the country's forces had seized a truck with eight rockets ready for launch and were pursuing the perpetrators of the attack on the military base.

It reiterated the "strong objection" to any aggression, whether from inside or outside Iraq, on Iraqi territories, interests and targets.

"We reject all reckless actions against Iraqi bases, diplomatic missions, and locations of the international coalition advisors, as well as anything that could escalate tension in the region or drag Iraq into dangerous situations," it added.

- Series of attacks -

Such attacks were frequent early in the war between Israel and Hamas Palestinian militants in Gaza, but until recently had largely halted.

The latest rocket fire comes as fears grow of an attack by Iran and its allies on Israel in retaliation for the killing of top Hamas and Hezbollah figures in strikes last week either blamed on or claimed by Israel.

The killings are among the most serious in a series of tit-for-tat attacks that have heightened fears of a regional conflagration stemming from the Gaza war.

The Iran-aligned "Axis of Resistance" against Israel, which also includes Iraqi groups and Yemen's Huthis, has already been drawn into the nearly 10-month war.

Monday's rocket attack came after US forces carried out a strike in Iraq last week that a US official said targeted combatants who were attempting to launch drones deemed a threat to American and allied troops.

The strike, which Iraqi sources said left four dead, was the first by American forces in Iraq since February.

There have also been two other recent attacks targeting Ain al-Assad base -- on July 16 and 25.

Prior to that, US troops in Iraq and Syria had not been targeted since April. But attacks against them were much more common in the first few months of the Israel-Hamas war in Gaza, when they were targeted more than 175 times.

The Islamic Resistance in Iraq, a loose alliance of pro-Iran groups, claimed the majority of the attacks, saying they were in solidarity with Palestinians in Gaza.

In January, a drone strike blamed on those groups killed three US soldiers at a base in Jordan. In retaliation, US forces launched dozens of strikes against Tehran-backed fighters in Iraq and Syria.

Baghdad has sought to defuse tensions, engaging in talks with Washington on the future of the US-led coalition's mission in Iraq, with Iran-backed groups demanding a withdrawal.

The US military has around 2,500 troops in Iraq and 900 in Syria.