Syrian government forces fired artillery shells into opposition areas of Homs on Sunday, as sporadic violence tested a fragile four-day-old cease-fire, even as the first United Nations monitors arrived in Damascus.

Violence levels in the 13-month-old uprising have dropped dramatically since a truce, brokered by U.N.-Arab League peace envoy Kofi Annan, took effect Thursday morning. Government forces have mostly stopped shelling opposition strongholds in restive cities throughout the country.

But sporadic violence has continued, and deep skepticism remains about the government´s commitment to the truce deal. Opposition activists said government security forces killed at least five people on Sunday.

Syria´s state-run SANA news agency accused rebels of killing four people, and quoted a senior officer pledging action to stop "armed terrorist groups from continuing their criminal aggressions," suggesting more violence is likely in coming days."The armed terrorist groups have hysterically escalated their aggressions on the army, the law-enforcement forces and the civilians," the SANA quoted a Syrian military official as saying.

A delegation of six cease-fire monitors, led by a Moroccan colonel, landed in Damascus late Sunday. An additional 25 to 30 U.N. monitors are expected to join them later this week, redeployed from existing U.N. missions in the region, said Ahmad Fawzi, a spokesman for Mr. Annan.

The monitors´ arrival stands as the best chance that the uprising might find a diplomatic resolution after months of escalating bloodshed. A vote Saturday by the U.N. Security Council approving the monitoring mission marked the first time the 15-member council had taken a unanimous stance on the conflict. Prior votes had been blocked by opposition from Russia and China.

But Mr. Annan´s six-point peace plan and the U.N. observer mission are off to uncertain starts. Government forces have refused to withdraw their forces and heavy weaponry from contested Syrian cities and return to their barracks, one of the six points agreed upon in Mr. Annan´s plan. The government has also maintained bans on peaceful demonstrations, violating another of the six points.

In addition, there are early hints of conflict surrounding themonitors´ mission. Much of the international community, including the U.S., Europe and their Arab allies, expects themonitors to be able to move about freely in the country without Syrian government minders. Russia, however, diluted the wording in the resolution, giving the Syrian regime some wiggle room on the issue.

Syrian government spokeswoman and presidential adviser Bouthaina Shaaban told the Reuters news agency on Sunday that the government couldn´t guarantee monitors´ security unless it was involved in "all steps on the ground," hinting that the Syrian government may be intent on sending its own minders to accompany the monitors in the country.

The Syrian government hasn´t yet agreed to a larger deployment of 250 monitors. U.N. Secretary-General Ban Ki-moon is to report to the Security Council by Thursday with plans for the deployment, which would require another Security Council resolution.

Mr. Annan´s team is also in discussions with the opposition about creating a united negotiating team to begin political dialogue with the government, another plank in Mr. Annan´s six-point plan.

If the peace holds, Mr. Annan hopes to press ahead with the dialogue in a bid to broker a permanent settlement to the conflict, which activists say has so far left more 10,000 people dead.

—Joe Lauria contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/16/12)