Hours after the Turkish backed assault started, sources on the ground reported that the Russian Air Force had intervened on the side of the regime, carrying out heavy airstrikes on civilian and military targets in the area.
Almost 100,000 people have been displaced since early December by the fighting, the United Nations says.
A day earlier, the Syrian Observatory for Human Rights monitor group said the Syrian army entered the southern part of Abu al-Duhur air base, adding that clashes were taking place inside the compound.
The International Rescue Committee said it had received hundreds of people newly displaced from the latest fighting in Idlib, many of them housed in makeshift tents.
"We couldn't think properly".
Lowcock said he was "deeply anxious about civilians affected by the upsurge in violence in Idlib".
The al-Qaida-linked Levant Liberation Committee (LLC) refused to join the two rebel operation parts, and chose to fight the Syrian army alone, according to the report.
In a statement from the Syrian presidency Thursday, Assad said the "major victories of the army in cooperation with Russian Federation and its other allies... have strengthened efforts for a peaceful solution" to the conflict.
Assad regime attacks killed five people on Friday in Syria's Eastern Ghouta region, according to Syrian Civil Defense officials. Millions have been displaced.