BEIRUT - Opposition activists accused Syrian troops of shelling two cities on Tuesday in a campaign to weaken forces fighting President Bashar al-Assad's government before a ceasefire deadline next week. Rebel fighters also kept up their attacks, killing three soldiers in separate actions in northern Syria, activists said. Assad has agreed to a ceasefire negotiated by international peace envoy Kofi Annan from April 10, the latest effort to end a year of bloodshed stemming from an uprising against his rule. An advance team from the United Nations' peacekeeping department will arrive in Damascus in the next two days to work out how observers can monitor the truce, Annan's spokesman said in Geneva. But Syrian opposition figures as well as Western governments have already made it clear they are not convinced that Assad, who has failed to honor previous commitments, would keep his word this time. "He is a liar," said Waleed al-Fares, an opposition activist in Homs, a city which came to symbolize the anti-Assad struggle as opposition-held areas endured weeks of bombardments and sniper fire.