The advance on the town, in rebel hands since 2014, threatens to encircle insurgents in their only patch of territory in nearby Hama province and could put a Turkish observation post there at risk.
Idlib province is the last major anti-Assad stronghold in Syria after eight years of war.
The escalation has killed at least 500 civilians and uprooted hundreds of thousands, many stranded near the border with Turkey, the United Nations says.
The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said airstrikes believed to be Russian struck near the highway and forced the almost 25-vehicle Turkish convoy to stop.
The government intensified its campaign in the country's northwest in April despite a de-escalation deal brokered by Damascus's ally Russian Federation and rebel-backer Turkey previous year.
Turkey, which hosts some 3.6 million registered Syrian refugees and warns it cannot accept more, fears the onslaught in Idlib could spark a new influx.
The source condemned the "act of aggression" by Turkey, and was cited as saying that the move would not impact "the determination of the Syrian Arab Army to keep hunting the remnants of terrorists".
Among those killed was Mohammed Hussein al-Qassem, a commander with the Failaq al-Sham rebel group that is supported by Turkey, opposition activists said.
It wasn't immediately clear who was behind the strikes, Syrian or Russian warplanes.
Turkey's private DHA news agency said Syrian government planes targeted the route of a Turkish military convoy carrying reinforcement vehicles and personnel.
Joshua Landis, who heads the Department of global and Area Studies at the University of Oklahoma, tells VOA that Khan Sheikhoun is "extremely important to the Syrian government" because "it sits astride the main highway linking Damascus to the north of the country". Damascus said the vehicles were transporting weapons and ammo to "terrorist forces".
The strikes hit near the highway where the convoy was traveling, Syrian activists said.
The army has closed in on the town from the east and west since a brief ceasefire collapsed earlier this month.