Syrian troops have killed dozens of civilians in the city of Hama, activists have said, as UN military observers toured protest centres near the capital Damascus, and both Brussels and Washington imposed new sanctions.Government forces attacked the Arbeen neighbourhood of the western city and its surrounding areas on Monday with light and heavy machine guns, killing 28 civilians, said the British-based Syrian Observatory for Human Rights.Video footage posted online by activists showed mortar rounds hitting the area, with plumes of smoke rising to the sky.According to some reports, the area was shelled before ground troops entered."It began in the morning with tanks and artillery, there were houses burning,” a local activist called Mousab told the Reuters news agency by telephone. "[Then] the military forces entered and shot people in the street.”Al Jazeera couldn’t independently verify accounts of violence due to restrictions imposed on international media.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.The UN-Arab League envoy Kofi Annan is due to brief the Security Council again on Tuesday.The deal has curbed some of the violence, but the alleged killings in Hama’s Arbeen district on Monday have laid bare the difficulty of bringing to a complete halt 13 months of fighting that the UN says has left more than 9,000 people dead.The Syrian government blames the violence on "armed terrorists” and said in February that more than 2,500 security forces personnel have been killed.Ceasefire ‘incomplete’Despite concerns over the mounting violence, Ban Ki-moon, the UN secretary-general, gave the approval for the deployment of 300 ceasefire monitors to Syria from next week.Ban insisted on Monday that the government of Bashar al-Assad, the Syrian president, ensure the protection of the unarmed observers and allow them to travel freely throughout the country.However, Lynn Pascoe, Ban’s political-affairs chief, told the Security Council that Assad’s compliance with a ceasefire plan "remains incomplete”.The UN observers visited several opposition-held suburbs near Damascus and were met by thousands of protesters demanding the collapse of the government.Monitors also visited the town of Zabadani, where government forces and opposition fighters have clashed repeatedly in recent months.An activist told Al Jazeera that "the regime wanted to take revenge” after residents had taken to the streets to meet them."They have arrested so many people across the city,” he said. "They haven’t been arresting people for some time, until today.”Activists’ videos showed monitors passing by army tanks posted in the streets, despite a call within Annan’s six-point peace plan for the withdrawal of armour from residential areas.However, Faisal Meqdad, Syria’s deputy foreign minister, stressed his government’s "total commitment to respecting the Annan plan,” adding that the "armed terrorist groups” - a reference to the opposition - had not yet accepted it.Nabil Elaraby, the Arab League chief, has called for a political process to resolve the crisis.