U.N. Secretary-General Ban Ki-moon is calling on Syria's government to maintain the fragile ceasefire in the war-torn country, hours before a team of international observers is set to arrive in Damascus.

During a meeting with Belgian Prime Minister Elio Di Rupo Sunday, Mr. Ban said he was urging “in the strongest possible terms that this cessation of violence must be kept.”

Despite the U.N.-brokered ceasefire, violence throughout the country has continued. On Sunday, Syrian forces heavily shelled rebel-held areas in the flashpoint city of Homs.

Activists in the Khaldiya neighborhood of Homs, one of several districts attacked by government troops, reported widespread mortar and heavy machine gun fire.

The head of the British-based Syrian Observatory for Human Rights, Rami Abdelrahman, said shells were being fired at a rate of three a minute and at least three people had been killed.

Elsewhere, rebels fighting to oust Syrian President Bashar al-Assad attacked a police station in northern Aleppo province.

In response, Syria warned that armed “terrorists groups” have intensified attacks in the country in conjunction with a recent internationally negotiated ceasefire. A military official quoted by state television said security forces “will prevent the terrorist groups from continuing their criminal attacks.”

A spokesman for U.N.-Arab League envoy Kofi Annan said six members of a 30-member advance U.N. monitor team would arrive Sunday. The group is to report on whether pro-Assad forces and opposition fighters are abiding by the terms of Mr. Annan's six-point peace plan.

Syrian Ambassador Bashar Ja'afari reiterated his commitment to the U.N. plan, which calls for a ceasefire, withdrawal of troops, dialogue between the government and opposition and a “political transition” for the country.

Saturday, the U.N. Security Council authorized the advance team and approved a larger contingent of up to 250 monitors once the situation has stabilized.

Russia, a strong supporter of Syria, said the peace plan was at a “critical juncture.” Ambassador Vitaly Churkin urged both sides to refrain from violence.

U.N. Secretary-General Ban Ki-moon said Saturday that “it is the government of Syria which has the primary responsibility to stop the violence and withdraw its forces.” He also announced an April 20 meeting in Geneva to discuss dealing with at least one million people displaced in Syria and neighboring countries.

The U.N. says more than 9,000 people have been killed in Syria's unrest over the past year.