The new head of the UN observer mission in Syria said on Monday that he saw heavy shelling in the central city of Homs and major damage to the nearby town of Rastan during his first visit into the field.

"During my visit to Homs, I was personally able to witness heavy shelling from artillery and mortars ongoing in the neighbourhoods of the city," Lieutenant General Babacar Gaye told reporters after Sunday's trip.

"Rastan was heavily damaged by an intensive shelling campaign and fierce fighting," Gaye said.

"There were damaged tanks left on the side of the streets, public infrastructure, such as bridges, was destroyed, and homes on the main roads inside the town were largely damaged."

Gaye said he had met both the provincial governor and members of the rebel Free Syrian Army to "gauge their readiness for local engagement and dialogue."

He called on both sides to exercise restraint and to move away from "this mindset of confrontation and militarisation."

He said he was "deeply concerned about the ongoing violence from both sides in Aleppo," Syria's most populous city, where an army assault on rebel positions entered its third day on Monday.

"My observers there have reported an upsurge in the violence, with helicopters, tanks and artillery being used," the Senegalese general said.

"It is imperative that both sides respect international humanitarian law and protect civilians."

Gaye took over as head of the troubled UN observer mission earlier this month after the Security Council on July 20 agreed a "final" 30-day extension to its mandate.

Gaye's predecessor Major General Robert Mood already announced a suspension of the mission's field operations in mid-June, in the face of escalating violence.

The mission's 300 military observers have since been reduced to just 150.

The mission was deployed to oversee a peace plan brokered by international envoy Kofi Annan, which was supposed to begin with a ceasefire in April that never took hold.