Islamic State group jihadists beheaded eight Syrian rebels who had surrendered in a town on the border with Iraq last week despite pledges of an amnesty, a monitor said Monday.

The Syrian Observatory for Human Rights said the men were executed and their bodies hung on makeshift crucifixes in Albu Kamal in the eastern province of Deir Ezzor.

The incoming head of Israel's Northern Command, Maj.-Gen. Aviv Kochavi, has warned that terrorist groups could join Hizbullah and Iran in carrying out attacks on the Jewish State.

“The radical axis of Iran, Syria and Hizbullah, which operated for many years to harm the state of Israel, may be joined by the threat of terror organizations that possess an extremist ideology and can change their goals and begin targeting the state of Israel,” Kochavi said on Sunday.

Canada conducted airstrikes on Islamic State positions in Iraq for the first time on Sunday, while reports emerged that the jihadist group had executed more than 200 tribespeople in recent days.

"Today's strike demonstrates our government's firm resolve to tackle the threat of terrorism and to stand with our allies against ISIL's atrocities against innocent women, children and men," Canadian Defense Minister Rob Nicholson said in a statement.

Nearly 2,000 people have died in Syrian prisons of torture, starvation and lack of medical treatment so far this year, the Syrian Observatory for Human Rights said on Sunday.

The Britain-based monitor said it had documented 1,917 deaths in Syrian prisons since the start of 2014, among them 27 children under the age of 18 and 11 women.

Lebanese Forces leader Samir Geagea has expressed concern that the rival Hizbullah-led coalition could be seeking a new political system in the country through its approach towards the stalled presidential vote, noting that the Sunni community in Lebanon is acting “under the pressure” of Hizbullah's fighting in Syria.

“Unfortunately, I don't see presidential elections in the foreseeable future,” Geagea said in an interview.

Jordan's King Abdullah II vowed on Sunday to oppose any Israeli attempt to change the status of Muslim or Christian holy sites in Jerusalem.

"Jordan will continue to confront, through all available means, Israeli unilateral policies and measures in Jerusalem and preserve its Muslim and Christian holy sites, until peace is restored to the land of peace," the king said in a speech.

With global attention focused on the fight against jihadists, Syria's regime has in recent weeks stepped up its use of deadly barrel bomb strikes, killing civilians and wreaking devastation.

In less than a fortnight, warplanes have dropped at least 401 barrel bombs on rebel areas in eight provinces, according to the Syrian Observatory for Human Rights, a monitoring group.

Fierce clashes erupted between Hizbullah fighters and al-Nusra Front jihadists in the Syrian Qalamoun along the Lebanese border, a monitor said Sunday.

The Syrian Observatory for Human Rights said Syrian regime troops backed by Hizbullah fighters engaged in battles with al-Qaida affiliate al-Nusra Front on the outskirts of the strategic Syrian al-Qalamoun region along the Lebanese border.

A newspaper close to Syria's regime criticized new U.N. peace envoy Staffan De Mistura for the first time Sunday over his call for ceasefire zones to allow for aid deliveries.

"De Mistura has been vested with a clear mission based on a solution to the crisis through negotiations between Syrians," Al-Watan wrote.

Al-Qaida affiliate Al-Nusra Front seized a town and several villages in Syria's Idlib province overnight, in another blow to Western-backed rebel fighters in the northwestern region, a monitor said Sunday.

The Syrian Observatory for Human Rights said Al-Nusra fighters captured the town of Khan al-Subul after the withdrawal of the Hazm movement, a moderate opposition group.