Syrian activists say Al Qaeda-linked fighters have captured at least three villages from Western-backed rebels in the country's northwest as the militants continue their push to assert control over an area once held by more moderate groups.

Nusra Front's advances expose the weakness of more moderate factions, which the U.S. hopes to forge into an effective fighting force.

An activist who uses the name of Alaa al-Deen says Al Qaeda fighters seized three villages -- Safuhan, Fatira and Hazari -- in northwestern Idlib province, and were pushing to take others.

The development comes a day after U.S. airstrikes hit two Nusra Front compounds in Idlib.

In the past week, the Al Qaeda affiliate has been overrunning strongholds in Idlib once held by two prominent rebel factions armed and trained by the United States.