US special forces carried out a mission in northwest Syria on Thursday, the Pentagon said, a raid which Syrian sources said was believed to have targeted an al Qaeda-linked terrorist.

Syrian rescue workers said at least 13 people including six children and four women were killed by clashes and explosions that erupted after the raid began, targeting a house in the Atmeh area near the Turkish border.

"US Special Operations forces under the control of US Central Command conducted a counter-terrorism mission this evening in northwest Syria. The mission was successful," Pentagon Press Secretary John Kirby said in a statement.

"There were no US casualties. More information will be provided as it becomes available."

He did not identify the target.

A number of terrorist groups with links to al Qaeda operate in northwestern Syria, the last major bastion of rebels fighting President Bashar al-Assad in the decade-long Syrian war. Leaders of the Islamic State group have also hidden out in the area.

Several rebel sources said they were almost certain the target was a leading terrorist, but could not be sure who.

Residents said helicopters landed and heavy gunfire and explosions were heard during the raid that began around midnight. US forces used loud speakers to warn women and children to leave the area, they said.

A video taken by a resident and seen by Reuters showed the bodies of two apparently lifeless children and a man in the rubble of a building at the location.

A witness said he saw several bodies at the scene. "There was blood everywhere," he told Reuters.

A rebel official who declined to be named said the terrorist who was the apparent target of the raid was with his family at the time.

Witnesses said the raid ended with aircraft, believed to be helicopters, leaving the site but unidentified reconnaissance planes were still hovering in the area.

Terrorist groups

Terrorists in northwestern Syria include Huras al-Din (Guardians of Religion), an al Qaeda-affiliated faction whose leaders include foreign fighters.

US forces have used drones to target the group and other terrorists in the area for years, but Thursday's operation appeared to be the largest of its kind by US forces in the northwest since Islamic State leader Abu Bakr al-Baghdadi died in a US special operations raid in 2019, said Charles Lister, senior fellow with the Washington-based Middle East Institute.

Islamic State fighters have been re-emerging as a deadly threat, aided by the lack of central control in many areas, according to a dozen security officials, local leaders and residents in northern Iraq.

The United States has offered rewards for information leading to the identification of senior Huras al-Din leaders.

The most powerful group in northwestern Syria is Hayat Tahrir al-Sham, the former Nusra Front, which was part of al Qaeda until 2016.

The rebel official said security from the Hayat Tahrir al-Sham group hurried to the location after the raid.

The Atmeh area is crammed with tens of thousands of Syrians who have been uprooted in the war and live in makeshift camps or overcrowded housing.