An advance team of six UN observers arrived in Damascus on Monday to monitor Syria's 4-day-old cease-fire. 25 other observers will join the team by setting up operating headquarters, and reaching out to the Syrian government and the opposition forces so that both sides fully understand their role. The UN action comes as President Bashar Assad's troops are firing tank shells and mortar rounds at neighborhoods in the opposition stronghold of Homs. Heavy clashes erupted Monday between opposition rebels and Syrian regular army units near the Turkish-Syrian border, as the first batch of a UN advance team were due to start observing the ceasefire. In the past four days, the opposition has registered 200 violations of the ceasefire by government forces. The Local Coordination Committees, which document violence across Syria, said 28 people were killed on Sunday.