The ambush is the second this week to target buses traveling between the regime-controlled areas and to be blamed on suspected Daesh militants.
On Saturday, air strikes blamed on Israel hit positions belonging to regime forces and Iran-backed militias near the border with Iraq, a war monitor said.
The UK-based Syrian Observatory for Human Rights said Israeli strikes on a military base run by Iranian-backed Lebanese Hezbollah caused the series of blasts.
Among the dead were four children while 16 others were wounded with some in critical condition, a war monitor says of the attack in the northwestern town, Sarmin.
Ten children are among the dead as a result of the regime air strikes and Russian military raids which hit a makeshift camp for displaced people in the town of Maarat al Numan in Idlib.
A war monitor says "fierce clashes" between loyalist forces, militants and allied rebels were taking place one kilometre west of Khan Shaykhun in Idlib province.
Anas al Dyab, a photographer and videographer in his early 20s, was a member of the White Helmets who also contributed to Turkey's Anadolu Agency.
Opposition factions seized Hamameyat village and a hilltop in Hama province amid deadly clashes, war monitor SOHR says.
Air strikes carried out by the Syrian regime and Russia targeted multiple towns in Idlib and Hama provinces.
Air strikes on the largely opposition and rebel-controlled region of northern Idlib followed air raids on Tuesday night and Wednesday that killed 23 civilians.
Britain-based war monitor Syrian Observatory for Human Rights says over 60 Syrian regime fighters have been killed in 48 hours.
Thousands of people, including militants, have left the last area held by Daesh in Syria, with the UN saying hundreds more are expected to arrive at camps for the displaced on Tuesday alone.
Subscribe to our Youtube channel for all latest in-depth, on the ground reporting from around the world.
Copyright © 2021 TRT World.