The Al-Qaeda affiliated Nusra Front and other rebel groups led a major offensive on Tuesday in northwest Syria, pushing back government troops in an area bordering President Bashar al-Assad's coastal heartland, a monitoring group said.

Since Monday night, the Army of Conquest alliance - known as Jaish al-Fatah in Arabic - has seized several positions from the Syrian army in Sahl al-Ghab, a large plain in Hama province bordering Latakia, bastion of the Assad clan.

In a video published by the alliance - which includes Nusra, Ahrar al-Sham, Faylaq al-Sham, a coalition of Muslim Brotherhood-linked battalions, Jund al-Aqsa, a small militant group - a commander boasts of the "liberation" of several strategic hilltops, checkpoints and a power plant.

The military leader also said "the army of Assad retreated to Alawite villages," referring to the sect to which Assad belongs.

According to the Syrian Observatory for Human Rights, a British-based monitoring group, the Army of Conquest's advance in Sahl al-Ghab, which also borders Idlib province, is highly significant.

"After this advance, there is nothing left for the regime in Idlib province - which is almost completely held by rebels - except the Abu Duhur airport and two Shiite villages, Fuaa and Kafraya," said Observatory head Rami Abdel Rahman.

"And in Hama province, the positions taken by the rebels were the first line of defence for several towns held by the regime as well as Alawite villages," he added.

Abdel Rahman said the first village likely to be targeted by rebels would be Jureen, where the Syrian army operates a command base for its forces fighting in Sahl al-Ghab.

The Army of Conquest seized control of Idlib city, capital of the province by the same name, on 28 March. After Raqqa, it was the second provincial capital to fall from the government's control.

Since then, the alliance has scored a series of major victories that puts it in control of a vast majority of the province, which borders Turkey.

Meanwhile, Syria's armed forces have withdrawn to reinforce their positions in Latakia in case of a rebel attack there.