DAMASCUS: Syrian troops killed 28 civilians in the city of Hama, monitors said, as UN military observers toured protest centres near the capital, and both Brussels and Washington imposed new sanctions.

The persistent bloodshed 11 days into a ceasefire sparked growing criticism from opposition activists of the fledgling UN mission, which still numbers just eight observers out of a planned initial deployment of 30.

Despite concerns over the mounting violence that the UN says has left over 9,000 people dead in 13 months of fighting, UN leader Ban Ki-moon gave the go-ahead for the deployment of 300 ceasefire monitors to Syria from next week.

Ban insisted Monday that the government President Bashar al-Assad ensure the protection of the unarmed observers and allow them to travel freely throughout the country.

His political chief B. Lynn Pascoe however told the UN Security Council that Assad's compliance with a ceasefire plan "remains incomplete."

Government troops strafed the Arbaeen neighbourhood of the central city of Hama and its environs on Monday with light and heavy machine guns, said the Syrian Observatory for Human Rights in reporting the 28 civilian deaths.

Video footage posted online by activists showed mortar rounds hitting the area, with plumes of smoke rising to the sky.

The UN observers visited several rebel suburbs near the capital and were met by thousands of protesters demanding the collapse of the regime.

Monitors also visited the town of Zabadani, where regime forces and rebel fighters have clashed repeatedly in past months.

Activists' videos showed monitors passing by army tanks posted in the streets, despite a call within UN-Arab League envoy Kofi Annan's six-point peace plan for the withdrawal of armour from residential areas.

However, Deputy Foreign Minister Faisal Meqdad stressed his government's "total commitment to respecting the Annan plan," adding that the "armed terrorist groups" - a reference to the rebels - had not yet accepted it.

Arab League chief Nabil al-Arabi called for a political process to resolve the crisis. (AFP)