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 defense official said on condition of anonymity.

The official added that the Central Command assessed that the drones "posed a threat to US and coalition forces".

"We maintain the inherent right to self-defense 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 -- said apparent missile strikes fired from drones hit two Hashed patrols in Babylon province.

An official from the alliance told AFP that "four people were killed and others were critically wounded". A security source confirmed the toll.

Kataeb Hezbollah, a powerful pro-Iran group and part of Hashed, said the US strike targeted a "group of drone experts who were carrying out new tests" to improve their surveillance drones.

The group added in a statement that it intended to use the surveillance drones to support Iraqi security forces next month during the Arbaeen commemoration -- one of the biggest Shiite religious events which marks the 40th day of mourning for Imam Hussein, the grandson of the Prophet Mohammed.

It also said "it would make the appropriate decision after consulting" with other groups, and urged the Iraqi government to "end the occupation" -- referring to the presence of US forces in Iraq.

Kataeb Hezbollah is also part of the Islamic Resistance of Iraq, a loose alliance of pro-Iran groups that had claimed attacks on US troops, most recently over the Gaza war, before suspending them in late January.

The United States considers Kataeb Hezbollah a "terrorist" group and has repeatedly targeted its operations.

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 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.

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, with Iran-backed groups demanding a withdrawal.

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 neighboring Syria.

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