US forces, backed by Arab allies, have launched their first airstrikes against militant targets inside Syria, reportedly killing both members of the self-declared Islamic State and those of another Al Qaeda-linked group.

The airstrikes, which involved jet fighters, drones, bombers, and cruise missiles, struck multiple targets in Syria early Tuesday. According to US Central Command (Centcom), the attacks involved support or participation from Bahrain, Jordan, Qatar, Saudi Arabia, and the United Arab Emirates. Reported targets included the IS stronghold of Raqqa, in northeast Syria, as well as training camps and checkpoints in at least four Syrian provinces.

Vox reports that airstrikes began about 2:30 am Tuesday morning, according to a Twitter user who live-tweeted the start of the bombing, just before the Pentagon’s confirmation of the campaign’s start.

The Syrian Observatory for Human Rights, a London-based monitoring group, said at least 20 IS militants were killed in the strikes, as well as 30 fighters for Jabat al-Nusra, Al Qaeda’s official arm in Syria and a rival of IS. At least eight civilians were also killed, the Observatory says.

One activist, known as Abu Khalil, told the Los Angeles Times that the strikes in Raqqa were focused on the provincial government offices, which serves as IS’s headquarters, and also struck former government military bases and a security building in the city. The activist said that IS fled the headquarters after the attack.

“These attacks will be answered,” an IS fighter in Syria told Reuters via Skype.

The US had informed the government of Syrian President Bashar al-Assad that the strikes would be taking place, according to Syria’s government. “The foreign minister received a letter from his American counterpart via the Iraqi foreign minister, in which he informed him that the United States and some of its allies would target [Islamic State] in Syria…. That was hours before the raids started,” it said, according to Reuters.

As well as targeting Islamic State, Centcom said airstrikes were launched to “disrupt imminent attack planning against the United States and Western interests” by “seasoned al Qaeda veterans.” Though Reuters reports that Centcom did not identify the group, other media outlets write that the target was the Khorasan group, a cadre of Al Qaeda veterans that poses “a more direct and imminent threat to the United States” than IS, according to US officials.

According to the Associated Press:

…The Khorasan militants did not go to Syria principally to fight the government of President Bashar Assad, U.S. officials say. Instead, they were sent by al-Qaida leader Ayman al-Zawahiri to recruit Europeans and Americans whose passports allow them to board a U.S.-bound airliner with less scrutiny from security officials.

In addition, according to classified U.S. intelligence assessments, the Khorasan militants have been working with bomb-makers from al-Qaida’s Yemen affiliate to test new ways to slip explosives past airport security. The fear is that the Khorasan militants will provide these sophisticated explosives to their Western recruits who could sneak them onto U.S.-bound flights. The Wall Street Journal reported last week that US intelligence officials were tracking Khorasan and that it worked closely with Jabat al-Nusra, the al-Qaeda affiliate. Al-Qaeda’s Pakistan-based leadership is at odds with IS, whose leaders refused to follow its orders and see Jabat al-Nusra as a rival rather than a partner in the Sunni extremist movement in Syria.

On Thursday, Director of National Intelligence James Clapper said Khorasan may pose as much of a danger as Islamic State “in terms of threat to the homeland.” It was the first time a U.S. official has acknowledged the group’s existence.

The groups have shown an affinity for bomb plots. Officials say they have grown alarmed that terrorists could attempt some attacks soon, such as a number targeting European countries from operatives based in Syria and Turkey.

The plots emanating from Syria likely have been under development for months, but the groups are vying for prominence with Islamic State, which has catapulted to the top of the U.S. target list in the region, the current and former officials said.

This post originally appeared in The Christian Science Monitor