Syrian regime forces unleashed an artillery barrage on a neighborhood of the city of Hama and executed residents who spoke with United Nations peace monitors, according to several residents, one day after protesters in the city took to the streets to welcome the observers.

Monday´s violence left 28 to 38 people dead in the city, according to reports by opposition networks.

"The regime decided to punish the people of Hama because they were brave enough to come out to meet the monitors," said Hama resident and opposition activist Mousab al-Hamadee. "The regime wanted to send a message to other cities."

The violence—among the worst in the city in several months, and perhaps the bloodiest day nationwide since a nominal cease-fire was to have taken effect on April 12—casts fresh doubt on the future of a monitoring mission proposed by the U.N., and on the ability of the observers to protect those with whom it comes in contact.

Eleven U.N. peace observers are now on the ground in Syria, said U.N. spokesman Eduardo del Buey, and some 20 more are due within days, tasked with monitoring the cease-fire deal brokered by U.N.-Arab League envoy Kofi Annan. A full contingent of 300 observers has been authorized by the U.N. Security Council.

The regime has continued shelling restive cities since shortly after the cease-fire went into effect, however, and has refused to pull its forces out of cities as pledged. The Syrian government´s adherence to the cease-fire is "clearly insufficient," B. Lynn Pascoe, the U.N.´s top political officer, told the U.N. Security Council on Monday. "Human-rights violations are still perpetrated with impunity."

But the U.N. appears intent on pressing ahead with the monitors´ deployment, as U.N.officials hope a more robust peace-monitoring team can help stem violence they say has now left 10,000 dead.

"The Syrian regime should make no mistake," said Susan Rice, the U.S. ambassador to the U.N., told the Security Council. "There will be consequences should the Syrian regime continue to ignore this council´s decisions."

But it isn´t clear what those consequences might be. Western government sought to include an explicit threat of sanctions in the council´s resolution on Saturday that authorized the 300 additional monitors. But in a compromise with Moscow, which opposes U.N. sanctions on Damascus, the language was toned down to say only that the council would consider "appropriate further measures" if Syria´s government doesn´t pull back its forces from populated areas in 15 days.

On Monday, the U.S. passed new sanctions that target telecommunications companies and government bodies in Syria as well as Iran that disrupt networks, monitor citizens or otherwise use technology to enable human-rights abuses.

U.K. Foreign Secretary William Hague said Monday the European Union was running out of credible sanctions against Syria. The latest ban on luxury-goods exports, passed Monday, represented the 14th round of EU sanctions against Syrian President Bashar al-Assad´s regime, including an oil embargo, an asset freeze and a travel ban on senior regime officials.

In Hama, Syria´s fourth-largest city and a hub of mass protests last summer, residents described artillery shelling, house raids and executions.

On Sunday, Mr. al-Hamadee and several other residents said, the U.N. observer team toured city neighborhoods speaking with residents. In many areas, they said, activists marched in the streets to greet the monitors, who left at around 3 p.m.

After the team left, security forces stormed into certain neighborhoods and began rounding up residents who had spoken to the observers, Mr. al-Hamadee said.

On Monday morning, Hama´s Arbaeen neighborhood, one of the districts the observers visited, suffered "very heavy, very indiscriminate" shelling for about four hours until 2 p.m., an activist in that neighborhood said in a conversation by Skype. Afterward, this person said, home raids and street executions began, with security forces and what residents characterized as plainclothes thugs entering the neighborhood and dragging people from their homes.

Nine young men were executed in the streets because they were alleged to have given information to the observers, Mr. al-Hamadee said. Another activist in the city said 12 people were executed. Those shot inside their homes or on the streets were civilians, rather than armed fighters with the Free Syrian Army, the activist in Arbaeen said.

Video footage posted on YouTube by Hama activist groups on Monday shows a room full of bodies of the alleged execution victims wrapped in white cloth shrouds with cardboard placards marking their names. Another video shows the bombardment of what is identified as Arbaeen, with the person shooting the footage saying: "This is the gift of the international observers and Kofi Annan to the Syrian people," as an explosion rocks the camera and smoke rises in the air.

The Security Council resolution on the monitors calls on the Syrian government to allow the observers to "freely and privately communicate with individuals throughout Syria without retaliation against any person" as a result of the interaction, the resolution says.

With outside media all but barred from Syria, the U.N. monitors are meant in part to provide independent confirmation of events there. At the U.N., Western diplomats said their governments were unable to confirm the events in Hama that were alleged to have followed the monitors´ departure.

The state-run SANA news agency, meanwhile, said rebel gunmen assassinated a doctor and an army officer Monday in the southern province of Daraa, and shot dead two other army officers in Hama.

—Nour Malas, Joe Lauria and Laurence Norman contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/24/12)