The Latest on the fighting in Syria (all times local):

3:15 p.m.

Russia's Defense Ministry says Washington's refusal to jointly fight al-Qaida's branch in Syria has contributed to the escalation of fighting in the war-torn country.

The ministry proposed last week that Russia and the U.S.-led coalition launch joint action against the Nusra Front, but the U.S. military said its contacts with Russia are only to maintain airspace safety in the crowded skies over Syria.

Lt. Gen. Sergei Rudskoi of the Russian military's General Staff said on Friday that the U.S. refusal to consider join action against the Nusra Front "leads to further escalation of the military conflict."

He also said the U.S. has failed to encourage opposition groups eager to abide by a U.S.- and Russian-brokered cease-fire in Syria to leave the areas where the al-Qaida affiliate is present, saying their failure to do so is threatening the truce.

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1:20 p.m.

Syrian activists say the Islamic State group has seized at least six villages from Syrian rebels near the Turkish border.

The rapid advances bring the militants closer to the border town of Azaz, where rebels hold an enclave that is hosting tens of thousands of internally displaced civilians.

In recent months, Syrian rebel factions in Azaz, which lies north of the city of Aleppo, have separately come under fire from the IS extremist group, pro-government forces and the predominantly Kurdish Syrian Democratic Forces.

The Britain-based Syrian Observatory for Human Rights says Friday's advance effectively cuts off a key supply route between Azaz and Marea, another opposition stronghold.

The IS group's news agency also reported the advance.