BEIRUT - Syrian government forces shelled the city of Homs on Sunday and rebels attacked a police station in Aleppo province, resident opposition activists and a rights group said, in more violence four days after a ceasefire was meant to come into effect.The fighting comes hours before an advance party of United Nations ceasefire monitors is due to arrive in Syria after Russia and China joined the rest of the Security Council on Saturday to authorise their deployment. "Early this morning we saw a helicopter and a spotter plane fly overhead. Ten minutes later, there was heavy shelling," said Walid al-Fares, an activist living in Khalidiya, one of the neighborhoods where mortars bombs have landed.Another resident said government loyalists were using heavy machine guns to shoot into the area.Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said shells were being fired at a rate of one a minute. He said there had also been overnight clashes in rural Aleppo."People said they heard explosions and shooting after rebels attacked a police station and then clashed with police," he said.A Syrian security source said on Sunday that "terrorist groups" have increased attacks in the country since an internationally brokered ceasefire to end violence was announced last week, state television said."Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life," state news agency SANA said.SANA said "armed terrorists" killed five people in ambushes around the country on Saturday. Although violence has persisted throughout the ceasefire, there has been a significant drop in the daily death toll in fighting which has often killed more than 100 people a day.On Saturday, 14 people were killed in the violence, Abdelrahman said and the state news agency SANA said "armed terrorists" killed five people in ambushes around the country.The United Nations Security Council voted on Saturday to authorize the deployment of up to 30 unarmed observers in the first resolution on Syria the 15-nation council has managed to approve unanimously since the uprising erupted in March 2011.Russia and China have previously blocked Western attempts to pass Security Council resolutions on Syria.A spokesman for UN-Arab League envoy Kofi Annan said on Saturday an advance team of six monitors would arrive in Syria within 24 hours and deploy within 36 hours, with more to follow within days."I will make sure that this advance observer mission will be dispatched as soon as possible and try to make concrete proposals by the 18th of April for an official observer mission," UN Secretary-General Ban Ki-moon told United Nations radio separately in Geneva.