Syrian troops deployed in support of Kurdish forces around a strategic northern city on Friday, in a shift of alliances hastened by last week's announcement of a US military withdrawal.
Northwestern Syria, where Idlib is located, is the last big area still in the hands of fighters seeking to overthrow President Bashar al-Assad.
The Syrian army has seized a chunk of territory from rebels in the southwest, Syrian state media and a war monitor said on Tuesday, the first major government advance in an offensive near the Jordanian border which has displaced tens of thousands.
The Syrian army has declared that all anti-regime forces have left Eastern Ghouta, after a blistering two month offensive on the rebel enclave on the outskirts of the capital.
The Syrian Observatory for Human Rights said there was intense fighting on several fronts accompanied by a government artillery barrage, continuous air raids and attacks by helicopters.
Israel also fired ground-to-ground rockets from the Israeli-occupied Golan Heights, but the Syrian defences brought the missiles down, the Syrian army said.
Former residents and the Syrian Observatory for Human Rights said there were heavy aerial strikes on eastern Syria's largest city as troops pushed towards the Hay al Umal area.
The separate assaults across the Iraqi-Syrian frontier aim to deal a final blow to the Islamic State "caliphate" that has crumbled this year in Syria and Iraq.
A ceasefire took effect on Sunday in an Islamic State pocket straddling the Syria-Lebanon border, where the jihadists have been fighting the Lebanese army on one front and Hezbollah with the Syrian army on the other.
The Lebanese group Hezbollah and the Syrian army launched an offensive on Friday to clear militants from an area of the Syrian-Lebanese frontier, a commander in the military alliance fighting in support of President Bashar al-Assad said.