The United States killed about 20 Al-Qaeda militants this week in air strikes in northwestern Syria, the Defense Department said Thursday.

US warplanes conducted two air strikes, Sunday and Tuesday, near Sarmada in Idlib province, the Pentagon said. The attacks had been reported by local sources.

According to those sources, the air strikes were aimed at Fateh al-Sham Front, a former Al-Qaeda affiliate, previously known as Al-Nusra Front, that officially severed ties with the jihadist network in July.

But for the Pentagon, the US assault near the city of Sarmada was aimed squarely at the "foreign terrorist fighter network" of Al-Qaeda.

Al-Qaeda leaders "directed terrorist operations out of this location," Pentagon spokesman Peter Cook said Thursday at a news conference.

"We will continue to take action to deny any safe haven for Al-Qaeda in Syria," he said.

Northwestern Idlib province is largely controlled by Fateh al-Sham and rebel groups allied with it.

Fateh al-Sham is excluded from the current ceasefire in Syria between the government and rebels that was brokered last week by regime backer Russia and rebel sponsor Turkey.

The tenuous ceasefire, which took effect on December 30, is part of efforts to resolve nearly six years of war in Syria, which has been ravaged by violence since an uprising against President Bashar al-Assad began in March 2011.

According to the Pentagon, the US air strike on Sunday targeted two vehicles and killed five Al-Qaeda militants.

Tuesday's strike killed more than 15 militants and destroyed six vehicles, Cook said.

"We will not allow Al-Qaeda to grow its capacity to attack the United States or its allies around the world," he said. "These strikes demonstrate that commitment."

The United States added Hamza bin Laden, son and would-be heir of Osama bin Laden, to its terrorist blacklist on Thursday.

The younger bin Laden has become active as an Al-Qaeda propagandist since his father's death at the hands of US special forces in 2011 in Pakistan.