BEIRUT, Lebanon — The Syrian government and its Russian backers have suggested that the recent takeover of the last rebel stronghold in Syria by an Al Qaeda-linked group could threaten a cease-fire that has been in place for several months.

Nearly a million of the more than three million civilians in Idlib Province have already fled their homes elsewhere in Syria, often more than once. Many landed there after Syrian President Bashar al-Assad’s government seized control of other opposition areas in recent years, busing rebels and civilians who refused to live under his rule to Idlib.

The shattering of the cease-fire in Idlib, in northwest Syria, would put the population in the path of yet another military onslaught and propel a wave of refugees into Turkey, which lies to the north.

Many analysts have regarded Mr. Assad’s assault on Idlib, backed by Russian air power, as a matter of when, not if. Mr. Assad is determined to retake control of all of Syria, grinding out victory after nearly eight years of civil war; Moscow is concerned about foreign fighters from its neighbors in Central Asia taking root in Idlib.