Syria's fragile ceasefire on the verge of collapse Two children were also among the dead, a spokesman said. The total number killed on Sunday was not yet clear by the end of the day. But Mr Fares said the attack on Homs was a warning to people not to talk to the UN observers when they arrived - in line with similar allegations made in advance of the arrival of Arab League observers in December. "They are attacking this area because there are too many activists there," Mr Fares said. They are teaching people a lesson, not to go and talk when the observers arrive. Most of the people died in buildings that collapsed from the shelling. We cannot pull them from the rubble, because there are snipers. Videos posted online purported to show both explosions and snipers on rooftops. The regime in turn accused rebel forces of attacking troops around the country, killing five people on Saturday, and said it would not refrain from retaliation. Rebel fighters reportedly attacked a police station in Aleppo province. "Authorities have a duty to safeguard the security of the nation and its citizens and will prevent these terrorist armed groups from continuing their criminal attacks," an official quoted by the state news agency said. Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life. The observers are supposed to be overseeing the ceasefire and the implementation of the six-point plan put forward by Kofi Annan, the former United Nations secretary-general and official envoy of both the UN and the Arab League. His spokesman, Ahmed Fawzi, said six observers were due to land on Sunday night, and would be augmented by 25-30 more "quickly." The aim is to muster a total force of monitors of up to 250. "Of course we are hoping that the process holds together until the observers get on the ground," he said. Ban Ki-moon, the UN secretary-general, said he was "very much concerned" about the situation. In a further threat to the mission, Bouthaina Shabaan, the media adviser to President Bashar al-Assad, said the authorities reserved the right to refuse admittance to observers based on their nationality - an indication that it will not co-operate with any representatives of countries the regime regards as hostile.