Isis has carried out its largest mass killing this year, executing 33 people, according to a monitoring group.

The terror group carried out mass execution in the al-Mayadin desert, near the strategic city of Deir Ezzor, the Britain-based Syrian Observatory for Human Rights reported.

It said its activists were "able to monitor the execution and see the bodies".

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The report said 33 people aged 18 to 25 were "slaughtered by sharp tools" near a hole dug by the militants, which was "filled with blood".

The Observatory said it was unknown whether those executed were Syrian government forces, allied militia or rebel fighters.

US President Donald Trump has said the United States and its allies will destroy Isis and "protect civilization."

"We will destroy Isis and we will protect civilization. We have no choice," Mr Trump said at a joint White House news conference with Jordan's King Abdullah.

He said the king has been a leader in the fight to defeat Isis "once and for all".

In pictures: Isis' weapons factories Show all 11 1 /11 In pictures: Isis' weapons factories In pictures: Isis' weapons factories A mortar round fin manufactured by Isis in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis rocket components discovered in Gogjali, Mosul, Iraq in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortars discovered near Karamlais, Iraq, in November 2016 CAR In pictures: Isis' weapons factories An Isis rocket launch frame in Qaraqosh, November 2016 Conflict Armament Research In pictures: Isis' weapons factories A memo from Isis' COSQC on quality control at a manufacturing facility in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Electrically-operated initiators manufactured by Isis in forces Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortar tubes at a manufacturing facility in Karamlais, November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis mortar production facility discovered in Gogjali, Mosul, in November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis weapons manufacturing facilities near Mosul in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Stocks of French-manufactured Sorbitol, Latvian potassium nitrate and Lebanese sugar at an Isis weapons factory in Iraq Conflict Armament Research In pictures: Isis' weapons factories A destroyed Isis weapons facility in Qaraqosh, Iraq, November 2016 Conflict Armament Research

The military campaign against the group has been building slowly. More cities have been recaptured from the militants, and Isis has lost large swaths of territory in Iraq and Syria.

However, the two biggest, climactic battles – to retake Mosul in Iraq, and Raqqa in Syria – have not been completed.

Last year Iraqi military forces, supported by the coalition, waged successful battles to oust Isis from a string of cities in the run-up to the Mosul fight.

Those included battles to take back eastern Mosul and the Iraq cities of Fallujah, Ramadi, and a number of smaller towns along the Tigris River. Backed by the coalition, the Iraqis were also able to defeat Isis at Makhmour and Qayyarah, enabling the military to establish bases at both locations and use them as logistical hubs for the battle to retake Mosul.

However, no major cities have been retaken in the last six weeks.

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In February, Iraqi forces launched what is expected to be the last major battle against Isis in Iraq — the campaign to retake western Mosul.

Success has been slower in Syria, where the US initially struggled to find a qualified cadre of local fighters to advise and train.