Syrian opposition activists said al Qaeda's affiliate in Syria, the Nusra Front, has seized large parts of the northwestern city of Idlib from government forces.

According to the Associated Press, they say the advances were made in fierce fighting Saturday, March 28, with the militants sweeping into neighborhoods close to the city center.

The Nusra Front is leading a major offensive that began earlier this week to take Idlib. If the city falls, it would be the second provincial capital lost by President Bashar Assad's forces in the four-year-old conflict.

A Syrian military official said army forces were fighting "fierce battles" against "armed terrorist groups" to regain control.

The Britain-based Syrian Observatory for Human Rights said militants seized control of most of Idlib after government forces withdrew to bases in the city.