Israel's army earlier said it had shot down a Syrian fighter jet that had infiltrated Israeli airspace, risking another escalation in the sensitive border zone.

Syrian troops have ousted rebels from a majority of the country's south, part of which borders the Israeli-occupied Golan, and are now closing in on a patch held by the Islamic State group.

The Damascus regime has long accused Israel of backing IS and other opposition factions.

"The Israeli enemy has confirmed its adoption of armed terrorist groups, and targeted one of our warplanes which was hitting them in the Saida region on the edge of the Yarmuk basin in Syrian airspace," the military source said.

The source, cited by state news agency SANA, did not specify if the plane had been hit or downed.

The town of Saida sits just inside Syrian territory, about one kilometre (less than a mile) east of the buffer zone with the Israeli-annexed Golan, and is held by IS.

Clashes were raging between regime troops and IS jihadists on the northern edges of the town on Tuesday, according to the Syrian Observatory for Human Rights, a Britain-based war monitor.

Israel has been on high alert for weeks as Syrian troops, backed by Russia, have drawn closer to the Golan.