Syrian activists say at least 24 rebels have died in battle for a military base near a northern opposition-held city but that government troops have managed to hold on to it.

The Britain-based Syrian Observatory for Human Rights says rebels fighting to topple President Bashar Assad's government launched an assault on the base northeast of the city of Raqqa on Wednesday.

Rami Abdurrahman, the Observatory chief, says sporadic clashes are still underway Friday for the complex, known as Base 17, and that at least 24 fighters have been killed in battle.

In February, Raqqa was the first city to fall entirely under rebel control since Syria's crisis erupted in March 2011.

At least 120,000 people have been killed in the conflict so far.