Al-Qaida in Syria seizes large parts of northern city of Idlib from government forces

In this image posted on the Twitter page of Syria's al-Qaida-linked Nusra Front on Saturday, March 28, 2015, which is consistent with AP reporting, a fighter from Syria's al-Qaida-linked Nusra Front holds his group flag as he stands in front of the governor building in Idlib province, north Syria. Al-Qaida's affiliate in Syria, the Nusra Front, captured most of the northwestern city of Idlib from government forces Saturday, sweeping into neighborhoods in the center of the city in a powerful blow to President Bashar Assad's government, opposition activists and the group said. (AP Photo/Nusra Front on Twitter) (The Associated Press)

In this image posted on the Twitter page of Syria's al-Qaida-linked Nusra Front on Friday, March 27, 2015, which is consistent with AP reporting, fighters from Syria's al-Qaida-linked Nusra Front take their positions as they fight against the Syrian government forces in Idlib province, north Syria. Al-Qaida's affiliate in Syria, the Nusra Front, captured most of the northwestern city of Idlib from government forces Saturday, sweeping into neighborhoods in the center of the city in a powerful blow to President Bashar Assad's government, opposition activists and the group said. (AP Photo/Nusra Front on Twitter) (The Associated Press)

In this image posted on the Twitter page of Syria's al-Qaida-linked Nusra Front on Friday, March 27, 2015, which is consistent with AP reporting, a fighter from Syria's al-Qaida-linked Nusra Front takes his position as he fights against the Syrian government forces in Idlib province, north Syria. Al-Qaida's affiliate in Syria, the Nusra Front, captured most of the northwestern city of Idlib from government forces Saturday, sweeping into neighborhoods in the center of the city in a powerful blow to President Bashar Assad's government, opposition activists and the group said. (AP Photo/Nusra Front on Twitter) (The Associated Press)

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

They say the advances were made in fierce fighting Saturday, 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.