BEIRUT, Lebanon — An intense and extended battle before dawn on Saturday between army defectors and Syrian security forces left at least 15 people dead in Idlib Province, a region in northwestern Syria that has proved to be one of the most restive since the uprising against President Bashar al-Assad began nearly nine months ago, a Syrian opposition group said.

The fighting in Idlib was the worst in a series of confrontations between government opponents, some of them peaceful, and security forces, in which at least eight other people were killed Saturday, the group said. The deaths came a day after the United Nations Human Rights Council urged tougher action against Syria over its crackdown.

The Syrian Observatory for Human Rights, based in London, said the gunfight in Idlib Province, near the Turkish border, lasted three hours. It said seven members of the security forces were killed, along with five army defectors. The group said that the fighting occurred near the provincial headquarters and that three civilians were killed in the cross-fire.

For weeks now, the Syrian Army security forces and insurgents, many of them army defectors, have waged a pitched battle in the province. Along with Dara’a, in southern Syria, and Homs, near the Lebanese border, Idlib has proved to be one of the government’s biggest challenges in an uprising that has turned increasingly violent amid a crackdown that the United Nations says has killed over 4,000 people.