U.N. Secretary-General Ban Ki-moon voiced concern Sunday over the continuing violence in Syria, urging the government to do everything possible to maintain the fragile cease-fire.

Following talks with Belgian Prime Minister Elio Di Rupo in Brussels, Mr. Ban said he was “very much concerned” about the shelling by government forces of rebel-held areas in the flashpoint city of Homs. He said he was urging “in the strongest possible terms that the cessation of violence must be kept.”

On Monday, Mr. Ban is scheduled to meet with top European Union officials.

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 five 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 cease-fire. A military official quoted by state television said security forces “will prevent the terrorist groups from continuing their criminal attacks.”

The violence continued despite the planned arrival in Damascus of a six-member advance U.N. monitor team. The group is to report on whether government forces and opposition fighters are abiding by the terms of Mr. Annan's six-point peace plan.

In Washington Sunday, Senator John McCain again called for arming Syrian rebels as part of a more robust effort to oust President Assad.

Appearing on CBS television's Face the Nation program, the Arizona lawmaker said America's response to continued bloodletting in Syria is “inadequate and shameful.” He said that “for the United States to sit and watch this wanton massacre is a betrayal of everything we stand for and believe in.”

Senator McCain recently met in Turkey with senior officers of the opposition Free Syrian Army, which has been pleading for foreign military assistance.

The Obama administration says it supports providing humanitarian relief to the Syrian people, and has backed U.N.-led efforts to halt the fighting and begin negotiations between Damascus and its opponents.

But McCain, the top Republican on the Senate Armed Services Committee, noted that Syrian rebels are outgunned and that – as he put it – it “is not a fair fight.”

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.

The U.N. estimates that more than 9,000 people have been killed since Syria's anti-government uprising began over a year ago.