BEIRUT — Syria’s military said Monday that it is determined to repel an attack on the government-controlled western part of Aleppo as it continued to battle insurgents in intense battles on the city’s edge.

The military said opposition fighters have killed 84 people, mostly women and children, since launching their offensive Friday. The Syrian Observatory for Human Rights, a monitoring group, estimates that 51 civilians, including 18 children, have been killed.

Rami Abdurrahman, the director of the Britain-based Observatory, which relies on a network of activists on the ground in Syria, said about 70 opposition fighters were killed in the fighting that included airstrikes on the frontline.

Secretary of State John Kerry on Monday vowed to work ‘‘to the last moment’’ to achieve a lasting cease-fire in Syria. In London to accept an award from the Chatham House think tank, Kerry said he would ‘‘rather be caught trying’’ to reach diplomatic agreements than submit to the inevitability of military conflict. He asserted that the nuclear deal he spearheaded with Iran had averted an almost-certain war.

Amnesty International said the opposition offensive in Aleppo was ‘‘marked by indiscriminate attacks on civilian areas.’’

Associated Press