International envoy Kofi Annan's spokesman says the advance team of six U.N. observers have arrived in Damascus to monitor Syria's 4-day-old cease-fire.

Ahmad Fawzi says the remaining 25 observers are expected to arrive days from now.

He said in a statement Monday the mission "will start with setting up operating headquarters, and reaching out to the Syrian government and the opposition forces so that both sides fully understand the role of the U.N. observers."

The U.N. action comes as the cease-fire seems to be eroding, with President Bashar Assad's troops firing tank shells and mortar rounds at neighborhoods in the opposition stronghold of Homs.

Though the level of violence has dropped, the attacks raise doubts about the cease-fire.