Syrian forces heavily shelled rebel-held areas in the flashpoint city of Homs Sunday, hours before a small advance team of unarmed U.N. observers is set to arrive in the war-torn country.

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

Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said shells were being fired at a rate of three a minute. Homs-based activists said the Bayada, Jouret el-Shayah, Qarabees and Qusour districts were also being shelled.

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

In a unanimous vote Saturday, the U.N. Security Council authorized an advance team of up to 30 unarmed military monitors to deploy to Syria in the coming days.

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

Saturday's resolution authorizes a larger contingent of up to 250 monitors once the situation has stabilized.

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

U.N. Secretary-General Ban Ki-moon reiterated in a statement Saturday that “it is the government of Syria which has the primary responsibility to stop the violence and withdraw its forces.”

He also announced that the U.N. will host a meeting in Geneva on April 20 to discuss dealing with at least 1 million people displaced inside Syria and in neighboring countries.

U.S. Ambassador Susan Rice cautioned that the advance team's deployment would be a test to see if it is possible to send a larger observer mission to Syria, once the secretary-general presents a blueprint, and if it is clear the truce is holding and Damascus is cooperating.

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.

Rights groups and activists said at least 11 people were killed by government troops across Syria Saturday. They said rebel fighters also appeared to be ignoring the truce in some areas.

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