Idlib province is nearly entirely held by Hayat Tahrir al-Sham, a militant outfit dominated by Al-Qaeda's former Syrian affiliate.
On Wednesday, 35 soldiers were killed at the airport, said the Observatory, which relies on a network of sources on the ground in the war-torn country.
Since losing the airbase in 2015 following a two-year jihadist siege, the Syrian government has only had control of two villages in Idlib province: Fua and Kafraya.
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.
The counter-offensive was unleashed by two rebel operation parts: the first consisted of the Turkey-backed Ahrar al-Sham, Jaish al-Ezzah, the Turkistan Party, and Nour Addien al-Zinki, while the second is formed of Jaish al-Naser, the Free Idlib Army, the Elite Forces and the Second Army rebels, opposition activists reported.
Since then, thousands of people have been fleeing with their belongings amid harsh cold weather toward safer areas further north, including Idlib city and areas near the border with Turkey. Air strikes across Idlib on Sunday reportedly left at least 21 people dead.
"We couldn't think properly".
Lowcock said he was "deeply anxious about civilians affected by the upsurge in violence in Idlib". Paris expressed concern about the humanitarian situation in that governorate and called for compliance with the agreements reached by the nations acting as the Syrian ceasefire guarantors (Russia, Iran and Turkey) in Astana in 2017 on setting up a de-escalation zone in Idlib.
Earlier, and demanded they "fulfil their responsibilities".
Turkey on Tuesday summoned the Russian and Iranian ambassadors to Ankara in order to convey its "uneasiness" about their actions in Idlib.
Turkey, which has been fiercely opposed to Assad, has recently been working with his allies Russian Federation and Iran for a political resolution to the conflict.
"Regime forces are striking moderate opposition with the pretext of fighting against Al-Nusra (Front)", Foreign Minister Mevlut Cavusoglu said, referring to the former Al-Qaeda affiliate.
More than 340,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests.
Some 2.65 million people are living in north-western Syria, including 1.16 million internally displaced people (IDPs), according to the UN.