The strike -- which Iraqi sources said left at least four people dead -- was the first by American forces in Iraq since February, when the US military said it killed a pro-Iran commander who was involved in attacks on the country's troops.

"Tonight, US forces in Iraq conducted a defensive air strike" in Babylon province "targeting combatants attempting to launch one-way attack uncrewed aerial systems", the US defence official said on condition of anonymity, adding that the Central Command assessed the drones "posed a threat to US and coalition forces."

"We maintain the inherent right to self-defence and will not hesitate to take appropriate action," the official added.

The Hashed al-Shaabi -- an alliance of pro-Iran former paramilitary groups that have been integrated into Iraq's security forces -- earlier said apparent missile strikes had left four people dead at a base in Babylon, but made no mention of an attempt to launch drones from the ground.

"Available information suggests that two patrols... were targeted with missiles fired from drones," the Hashed said in a statement.

An official from the alliance told AFP that "four people were killed and others were critically wounded in blasts that hit a base belonging to the Hashed al-Shaabi".

A security source confirmed that four people were killed, adding that the death toll was expected to rise.

- Repeated attacks -

There have been two recent attacks targeting US and allied forces in Iraq.

Several rockets were launched toward Ain al-Assad base last week, which hosts troops form the US-led coalition against the Islamic State jihadist group, officials said.

And on July 16, two drones were launched toward the same facility, one of which was "successfully destroyed" while another hit the base, causing "minimal damage," according to the Pentagon.

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 Iran-backed 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.

Since then, attacks against US troops have largely halted.

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

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

The coalition was deployed to Iraq at the government's request in 2014 to help combat the Islamic State group, which had taken over vast swathes of Iraq and neighbouring Syria.

IS remnants still carry out attacks and ambushes in both countries.