Syrian rebels seized control Thursday of the Islamic State group's main supply route to Turkey, a monitor said.

"Rebel factions and Islamists took control of the northeast of Al-Rai," a town occupied by IS on the border between Syria and Turkey, the Syrian Observatory for Human Rights said.

"This is the main and one of the last crossing points with Turkey."

Observatory director Rami Abdel Rahman told AFP that rebels entered Al-Rai on Thursday following two days of clashes.

According to Abdel Rahman, the jihadists still control a crossing point further east, in the town of Halwaniyeh, but "Al-Rai was where they mainly smuggled in jihadists, whereas Halwaniyeh is reserved for top commanders".

IS has suffered a string of setbacks in recent months, including the loss of the ancient city of Palmyra, east of Damascus, to pro-regime forces in March.

A ceasefire that came into effect on February 27 has drastically reduced violence across Syria, but areas controlled by IS, the Al-Qaeda affiliated Al-Nusra front, and other jihadist groups were exempt from the truce.

Abdel Rahman said that IS had lost control of at least 18 villages in the northern province of Aleppo in recent days.

Since Syria's conflict erupted in March 2011, thousands of people have gone missing -- many of them arbitrarily arrested by armed forces -- across the country.

More than 270,000 people have been killed and millions have fled their homes.

UN-backed peace talks to bring an end to the conflict are set to resume next week in Geneva.