If it is recaptured, the airport will be "the first military base in the hands of the regime in Idlib province", said Observatory director Rami Abdel Rahman.
Russian-backed regime forces launched an offensive at the end of 2017 on the edge of Idlib province, the last in the country still fully outside the government's control.
The al-Qaida-linked Levant Liberation Committee (LLC) refused to join the two rebel operation parts, and made a decision to fight the Syrian army alone, according to the report.
Rebels fighting under the banner of the Free Syrian Army (FSA) said in a statement they had set up a joint operations room to repel the offensive and take back areas seized by the government in northeastern Hama and southern Idlib.
The presence of militants within southwest Aleppo is confirmed by the fact that Syrian army-led forces advancing towards government positions in eastern Idlib via Khanasser have encountered resistance from Hayat Tahrir al-Sham fighters.
The regime lost control of the Abu Duhur airbase in 2015 after a two-year siege by jihadists, with only the Shiite villages of Fuaa and Kafraya remaining under its control in the whole province.
Meanwhile, the Syrian Foreign Ministry denied claims by the French Foreign Ministry about the Syrian army's targeting of hospitals and civilians during the military campaign in Idlib.
Almost 100,000 people have been displaced since early December by fighting in northwestern Syria, where the regime is waging an offensive against jihadis, the United Nations said on Wednesday.
Turkey has accused the Syrian government of using the presence of the Nusra Front, which now fights under the banner of the Tahrir al-Sham alliance, as an excuse to attack civilians and moderate opposition groups.