Al Qaeda's Nusra Front has made significant gains in the Idlib province in recent months. Above, members of the group walk along a street in the northwestern city of Ariha after a coalition of insurgent groups seized the area May 29, 2015. REUTERS/Abed Kontar

Al-Nusra Front, al Qaeda’s Syrian affiliate, has seized control of the last military base held by government forces in the northwestern province of Idlib, a monitoring group said Wednesday, according to the Al Arabiya News. The gain marked a major success for Islamist rebels, who have laid siege to the Abu Duhur military airport for the last two years, the Syrian Observatory for Human Rights said. Syrian state television confirmed “the retreat of the army” from the airport Wednesday.

The loss of most of the base was a strategic blow to the Syrian military, defense analyst Obai Shahbandar told Al Jazeera. "The rebel strategy seems to be to encircle the Assad regime's forces and force them to withdraw. ... This was certainly a military and political embarrassment for Bashar al-Assad," Shahbandar said.

A continuing sandstorm in the region allowed rebel fighters to advance on the site, gaining the upper hand, as regime forces could not shell accurately by plane, Al Jazeera English reported. Not all military forces had withdrawn, and clashes reportedly continued in the area Wednesday.

Al-Nusra fighters, joined by a collection of other rebel groups known as the Fatah Army, have made gradual gains on the airport in recent months. Rebels managed to seize the entrance to the base and several positions on its outskirts in August by sending suicide bombers by motorbike.

Al-Nusra and other Islamist rebels are now in control of much of the Idlib province. Rebels drove out government forces from the provincial capital in March -- considered a major win for al Qaeda in Syria.

Al-Nusra is listed as a terrorist organization by Washington, and its spread in recent years has been a source of concern for Western military experts. The extremist group’s strength, paralleled with the rise of the Islamic State, also known as ISIS, has led to the weakening of moderate opposition groups supported by the United States and other Western countries.