BEIRUT, Lebanon — As many as 100,000 people have been forced to flee their homes in northwestern Syria as the government of President Bashar al-Assad and his patron, Russia, have intensified a military offensive there in recent days, aid groups said.

Trapped between the violence and a closed Turkish border, the civilians are huddling in makeshift settlements that, in many cases, lack toilets or clean water.

There is no foreseeable end to the suffering for the roughly three million residents of Idlib Province, which has been slipping out of the Syrian rebels’ grip, airstrike by airstrike, skirmish by skirmish, bringing Mr. Assad increasingly close to controlling the entire country again after more than eight years of civil war.

To the north, Turkey has sealed its border, keeping displaced civilians out. Other international powers have long since stopped efforts to intervene against the government’s advances, even as they call for political change in Damascus.