U.N.-Arab League envoy Kofi Annan is holding out hope that Syria's government and opposition will abide by terms of a cease-fire plan that he brokered, as a Tuesday deadline for Damascus to begin pulling its forces out of urban areas came without a cessation of attacks.

The deal brokered by Mr. Annan said Syria's government must begin pulling troops out of population centers by Tuesday morning, with a full cease-fire by both sides within 48 hours. But hopes for the plan dimmed after a fresh wave of violence and new demands by the government for written guarantees that the opposition will lay down arms first.

Mr. Annan said Tuesday that Syria has until April 12 to fully implement the cease-fire. He added: “it is a bit too early to say that the plan has failed.”

The Syrian Observatory for Human Rights says at least 17 people, mostly civilians, were killed in violence on Tuesday. The London-based group says some of the deaths came from Syrian government shelling in the central Hama region and the northwestern town of Mareh.

Syrian opposition activists in Geneva say at least 1,000 people have been killed by government forces in the last eight days.

In a Tuesday news conference from Geneva, spokeswoman Basma Kodmani of the opposition Syrian National Council said there were no indications that the government of President Bashar al-Assad was honoring terms of the cease-fire, and she said the opposition group could not accept a partial withdrawal of government forces.

Turkish Prime Minister Recep Tayyip Erdogan on Tuesday accused Syrian government forces of violating the border and said his country is considering what steps to take in response, including measures “we do not want to think about.” He did not elaborate.

Mr. Annan commented from Turkey, where he visiting border camps set up for Syrians fleeing the violence in their country. On Monday, Syrian forces fired shots across the border, wounding six people at one of the camps. Turkey is giving shelter to about 24,000 Syrian refugees.

In Moscow, Syria's Foreign Minister Walid al-Moallem said his government had begun to fulfill Mr. Annan's plan to end the violence. But he seemed to raise another new demand, saying a cease-fire must start simultaneously with the deployment of an international observer mission.

Moallem's Russian counterpart, Sergei Lavrov, said the Syrian government “could have been more decisive” in implementing the peace plan, but he also called on opposition forces to halt violence. Russia has been one of few world powers to offer some support to Mr. Assad during his bloody crackdown on protesters.

U.N. officials say more than 9,000 people have been killed in Syria since the uprising began 13 months ago.