BEIRUT, Lebanon — Syrian government troops are pushing into a strategic town in the country’s last rebel-held region, local witnesses and monitors said Tuesday, another milestone in a military campaign that has trapped about three million civilians in the area.

After Russian warplanes pounded it for hours overnight, Syrian government troops and allied militias appeared close to seizing the town, Khan Sheikhoun, which lies on an important transportation route in western Syria. Government control of the town would tighten the siege on Idlib Province, which government forces have been assaulting since April.

Rebels retreated to the town’s outskirts on Tuesday, though clashes continued. The main rebel group among the local fighters, Hayat Tahrir al-Sham, portrayed the withdrawal as a “redeployment” and vowed to continue defending the town.

A fighter with the group, reached on WhatsApp, said the government had captured western and northern quarters of the town while rebels held the east and south.