Activists say Syrian forces killed at least seven people on Wednesday as a small group of U.N. observers resumed their mission to monitor a shaky truce in the government's year-long conflict with rebels.

The Britain-based Syrian Observatory for Human Rights says four people were killed and several wounded when security forces opened fire on a bus at a government checkpoint in the northern province of Idlib.

Syria's state-run SANA news agency says security forces killed one “terrorist” in Idlib province after stopping an “attempted infiltration” of armed militants from Turkey. It is unclear if the report was referring to the same incident.

Rights groups also say security forces killed two people in suburban Damascus and that a child died after being struck by gunfire in the eastern region of Deir el-Zour.

The Associated Press said four more U.N. monitors arrived in Damascus, joining 11 who began the observer mission last week.

The U.N. Security Council has approved an expansion of the mission to 300 personnel. In a Tuesday briefing, peacekeeping chief Herve Ladsous predicted 30 observers would be in place by the end of the week and the number would reach 100 within a month.

The observer mission is part of a peace plan brokered by international envoy Kofi Annan. He told the council Tuesday that the situation in Syria “continues to be unacceptable.” Mr. Annan expressed concern about media reports that Syrian troops have attacked civilian areas following visits by observers.

U.N. diplomats quote Mr. Annan as saying he is aware of reports that Syrian troops fired automatic weapons and killed a significant number of people in the central city of Hama on Monday.

The United Nations estimates that more than 9,000 people have been killed in Syria's 13-month crackdown on the revolt, while activist groups put the death toll at more than 11,000.