BEIRUT, Lebanon - Syrian troops armed with heavy machine guns killed dozens in the central city of Hama on Monday, activists said, just a day after chanting protesters welcomed a visit by a U.N. team sent to observe a shaky cease-fire.

The day's violence, the city's worst in months, added a dangerous new aspect to the U.N. team's work: that the Syrian regime might exact deadly revenge against opponents who feel empowered by the observers' presence to spill into the streets.

Observance of the truce, which was supposed to begin April 12, has been spotty at best. The main manifestation has been a temporary halt to fighting between President Bashar al-Assad's troops and rebel forces in locations where observers are present.

Skepticism about the cease-fire remains high, but world leaders say the current plan to stop more than a year of hostilities might be the last chance.

The U.N's political chief, B. Lynn Pascoe, told the Security Council that the Syrian government is still using heavy weapons and has failed to implement the peace plan brokered by former Secretary-General Kofi Annan.

Pascoe said Syria's compliance with other elements of the plan, like releasing detainees and allowing peaceful demonstrations, is "clearly insufficient."

Some details of Monday's events in Hama remained murky, though different activists said the attacks started early in the morning and stopped a few hours later after dozens of people had been killed.

Rami Abdul-Rahman, the head of the Britain-based Syrian Observatory for Human Rights, said troops stormed the town, firing assault rifles and heavy machine guns, killing at least 33 people.

An activist named Ahmed, reached by phone in Hama, said troops shelled the Arbeen district early Monday before sending in foot soldiers. He said 27 people were killed, and families were still searching damaged homes for others. He did not give his full name for fear of retribution.

Another activist in Hama, Mousab Alhamadee, reached via Skype, said as many as 50 had been killed. He guessed the attack was a direct result of the U.N. monitor's visit Monday.

"This was the punishment for the people of Hama because yesterday they were very brave when they met the U.N. monitors," he said.

In Washington, under pressure to stop the Syrian crackdown, President Obama on Monday levied new sanctions on people and entities in Syria and Iran that use technology to target their citizens and perpetrate human rights abuses.

His announcement underscored the degree to which technology, from cellphones to social media, has fueled popular uprisings in countries throughout the Arab world and at the same time has given autocratic regimes new ways to track dissidents and suppress political dissent.

The European Union also took action Monday, banning the sale of luxury goods and products to Syria that can have military as well as civilian uses.