Idlib province is nearly entirely held by Hayat Tahrir al-Sham, a jihadist outfit dominated by Al-Qaeda's former Syrian affiliate.
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. The attack also aims to secure the road liking the capital, Damascus, with the northern city of Aleppo, Syria's largest.
Almost 100,000 people have been displaced since early December by the fighting, the United Nations says.
The fighting and air strikes have forced more than 60,000 people to leave their homes since November 1, according to the UN Office for the Coordination of Humanitarian Affairs (OCHA). By the end of last week, they had advanced into Idlib, close to an insurgent-held military airport.
"We couldn't think properly".
Ninety-six civilians, including 27 children, have been killed since December 25 in Syrian or Russian air strikes on Idlib, according to the Observatory.
Iran says nuclear deal with major powers not renegotiable
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Lowcock said he was "deeply anxious about civilians affected by the upsurge in violence in Idlib".
In May past year, Russia, Iran and Turkey signed an agreement to set up de-escalation centres in Syria.
The regime's Russian-backed offensive has angered Ankara, which on Wednesday called on Moscow and Tehran to "fulfil their responsibility" and halt the operation.
Syrian state media, citing a foreign ministry source, said the French foreign ministry had shown "great ignorance about what was happening in rural Idlib province", which it said the army was fighting to "liberate from the terrorism of the Nusra Front and the other terrorist organisations that belong to it".
The Turkish government said the Idlib offensive was endangering the effort to reach a resolution of the conflict and accused the Syrian government of using the pretext of fighting militants to wage a widescale attack. Millions have been displaced.