BEIRUT, Lebanon — Multiple airstrikes hit a busy market in a rebel-controlled town in northwestern Syria on Monday, killing at least 27 people and turning several buildings into piles of rubble, according to opposition activists and a war monitor.

Shortly after the strikes, state media said rebels shelled a government-held village, killing seven.

The high death toll marked a sharp increase in the escalation of violence between the two sides. Government troops, backed by Russian air cover, have been trying since April to push their way into the enclave in the northwestern corner of Syria, near the Turkish border.

Dominated by Al Qaeda-linked militants and other jihadi groups, Idlib province and northern parts of the nearby Hama region are the last major rebel strongholds in the country not controlled by Syria’s president, Bashar al-Assad.