BEIRUT — Syria’s al-Qaida affiliate overran bases belonging to a Western-backed rebel group near the northern city of Aleppo after heavy clashes that killed at least 20 fighters, activists said Saturday.

The fighting happened near the town of Atarib some 18 miles west of Aleppo, and pitted the al-Qaida-linked Nusra Front against the mainstream Hazm Movement, which has received support from the United States.

The Britain-based Syrian Observatory for Human Rights said Nusra Front fighters seized control of several Hazm bases. At least 29 Hazm fighters and six Nusra Front militants were killed in the clashes, it said.

An Aleppo-based activist who goes by the name Abu al-Hassan confirmed the clashes, but put the death toll at around 20 Hazm fighters. Abu al-Hassan said by Skype that the two groups have had “numerous disagreements” in recent weeks.

In November, the Nusra Front expelled a once-powerful mainstream rebel group from Syria’s northern Idlib province. The al-Qaida group frequently cooperates with more moderate rebel factions, but has turned more hard-line since the U.S. targeted Nusra Front fighters with airstrikes as part of its wider campaign against Islamic extremists in Syria and Iraq.

Also Saturday, U.N. envoy Staffan de Mistura arrived in Damascus for talks with Syrian officials about his plan for a “freeze” on fighting in Aleppo. He envisions a local truce in Aleppo as the first step toward a wider easing of hostilities.