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.
It added that the Syrian forces have fought intense battles against the militant groups, including the al-Qaida-linked Levant Liberation Committee (LLC), in both countrysides over the past few hours.
The Syrian government hit back on Thursday against French criticism of its military campaign in rebel-held Idlib, saying it was targeting terrorist groups that were not party to an agreement to reduce fighting in the rebel-held region.
Tahrir al-Sham (HTS), an alliance led by the Nusra Front, the dominant force in Idlib, said it had already made gains.
Idlib province is the biggest chunk of Syria still held by rebels fighting President Bashar al-Assad, with a population swollen by Syrians who have fled government advances in other parts of the country.
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Rebels launched a counter attack on Thursday.
The activists said the warplane belonged to the Syrian army. Already, some strikes were reported in Idlib towns.
The statement also denied the army had targeted civilians or hospitals, as alleged by France.
Director of the Extremism and Counterterrorism Programme at the Middle East Institute, Charles Lister tweeted yesterday that major factions including Ahrar Al-Shaam, Nour Al-Din Al-Zinki and the Free Idlib Army were recipients of the weapons, but not the largest opposition conglomerate and former Al-Qaeda affiliate, Hayaat Tahrir Al-Shaam.
"The brigades killed more than 50 regime soldiers and captured 17 so far", Mohammad Rashid, a rebel fighter and spokesman for Jaish al-Nasr, told The Times.