Nato Secretary General Jens Stoltenberg said on Monday that a fragile truce in Syria appeared to be largely holding but the alliance was concerned by a Russian military build-up in Syria.

France, though, said it had information about fresh attacks on zones held by moderate rebels, and called for an immediate meeting of the Syria task force to address breaches of the cessation of hostilities that came into force on Friday.

The cessation of hostilities deal is the first of its kind to be attempted in four years. Syria's conflict began in 2011.

Stoltenberg told a news conference in the Gulf Arab state of Kuwait: "We have seen some encouraging developments that the ceasefire is largely holding but at the same time we have seen some reports about violations of the ceasefire.

"This agreement and the full implementation of the agreement is the best possible basis for renewing the efforts to find a political negotiated peaceful solution to the crisis in Syria," he added.

The deal, which is less binding than a formal ceasefire and was not directly signed by Syria's warring government and rebel forces, does not cover action against militants from Islamic State or the Nusra Front, an al Qaeda affiliate.

Syrian President Bashar Al Assad and his big power ally, Russia, say they will continue to fight militant forces. Other rebels say they fear this stance may be used to justify attacks against them too.

"We are concerned," Stoltenberg said, "about the significant Russian military build-up we have seen in Syria with the ground troops, with the naval forces in eastern Mediterranean and with air forces conducting air strikes."-Reuters