BEIRUT — A day ago, crowds in the Syrian city of Hama welcomed a U.N. team sent in to observe a shaky truce. On Monday, government troops opened fire on the same streets, killing dozens, activists said, raising fears the regime is targeting opponents emboldened to protest by the U.N. monitors.

President Obama and European countries announced new sanctions against Damascus, while the U.N.’s political chief said the Syrian government has failed to implement the peace plan designed to end 13 months of deadly conflict that has killed more than 9,000 people.

The worst violence in Hama in months came despite the cease-fire that went into effect April 12. Skepticism about the commitment to the truce by Syrian President Bashar Assad remains high among the regime’s opponents and some of the peace plan’s key backers.

U.N. political chief B. Lynn Pascoe told the Security Council that the Syrian government is still using heavy weapons against its people and has failed to implement key parts of the plan, such as releasing detainees and allowing peaceful demonstrations. The cease-fire is supposed to allow for dialogue on a political solution between Assad’s regime and those seeking his ouster.

“Human rights violations are still perpetrated with impunity,” Pascoe said.

The U.N. has sent an advance team of 11 observers to Syria to push forward the peace plan put forth by envoy Kofi Annan. More monitors are due to be on the ground by the end of the month, the U.N. said, part of a mission of 300 total.

While deaths nationwide dipped in recent days, the violence in Hama and elsewhere Monday suggested the regime was attacking those who voiced grievances to the observers.

“This was the punishment for the people of Hama because yesterday they were very brave when they met the U.N. monitors,” activist Mousab Alhamadee said via Skype.

He said government troops drove through the Musha al-Arbeen neighborhood on the city’s northeast edge, firing automatic weapons and killing at least 32 people. Amateur video posted online showed protesters near the observers’ cars Sunday in the same area, chanting, “Long live Syria! Down with Assad!”