Speaking in Damascus to Britain's Sky News on Wednesday, in his first television interview in the role, Major General Robert Mood brushed off criticism that the United Nations Supervision Mission in Syria (UNSMIS) had been too slow to get going.

The Norwegian said their numbers of boots on the ground would double in the coming days.

"This is not easy and we are seeing—by the action, by explosions, by firing—that the ceasefire is really a shaky one. It's not holding," the 53-year-old said.

"But what we are also seeing on the ground is that where we have observers present, they have a calming effect and we're also seeing that those operating on the ground, they take advice from our observers."

Syria's army on Wednesday reportedly suffered its deadliest day in a ceasefire when rebel fighters killed 20 troops, in the latest violation of the three-week truce the UN says both sides are flouting.

Mood said he would have 59 people in place in Damascus by the end of Wednesday and aimed to double the figure in the coming days as more flights come in carrying troops, vehicles and equipment.

"The people and equipment now on the ground is actually exactly how we want to see it, so the next couple of days—picking up the pace, doubling and spreading out—is exactly what suits us very well," he said.