(AP) — 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.

U.S. President Barack 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 new bloodshed – the worst violence in the central city of 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, such as the United States.

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!”

Another activist reached by phone said the troops shelled before opening fire, killing at least 31. Residents were still searching for others, said Ahmed, declining to give his full name for fear of retribution.

“Those observers brought destruction upon us,” he said. “Any area they visit the regime attacks. It’s a tragedy.”

Rami Abdul-Rahman, head of the Britain-based Syrian Observatory for Human Rights, said the day’s violence started when local rebels attacked the car of an army officer, killing him and his assistant. Regime forces later stormed the town, killing 33 people, he said.

Protesters elsewhere were also attacked after receiving observers. Thousands chanted and danced around the observers’ cars in Douma, near the capital of Damascus, only to face gunfire and tear gas from security forces when the observers left, according to activists and amateur video.

Observers also went to the Damascus suburb of Zabadani, where they talked to a few people and saw buildings damaged by government attacks.

Local activist Fares Mohammed said tanks that had been posted in the town center withdrew hours before the visit to an area less than a half-kilometer (one mile) away. Observers declined offers by residents to show them the place, he said. “Those tanks can be back in the city in two minutes.”

Two of the observers have taken up residence in an upscale hotel in the central city of Homs, bringing about three days of relative calm. Before the observers arrived, government forces had shelled the city for months.

“There is a big difference,” said Homs activist Abu Mohammed Ibrahim via Skype. “Before, we were getting hit with rockets and mortars. Now there are snipers and some gunfire, but only medium weapons. Before they fired all they had at us.”

Ibrahim said local rebels were observing the cease-fire, avoiding military checkpoints and streets where the government had posted snipers.

Also Monday, a Jordanian relief agency said Syrian troops ambushed hundreds of people fleeing the country over the weekend, and dozens of them crossed into Jordan with burns and gunshot wounds. The Kitab and Sunna charity said Syrian forces detained dozens of people, including about 50 women.

In Washington, Obama announced new sanctions on bodies in Iran and Syria that use technology to target citizens by blocking or monitoring social networking sites used to organize demonstrations and communicate with the media.

“National sovereignty is never a license to slaughter your people,” Obama said.

In Luxembourg, the EU passed its 14th set of sanctions, this time banning “luxury goods” and products that can be used against protesters.

The new sanctions are aimed at Syria’s wealthy business class, which has largely stood behind Assad.

Emails purportedly from Assad and his wife, Asma, published in February by London’s Guardian newspaper, indicated the Syrian first lady has a taste for fine goods, and had shopped online for crystal-encrusted Christian Louboutin stilettos, expensive jewelry, custom-made furniture and other luxury items as violence swept the country.

Previous rounds of U.S. and EU sanctions have done little to stop the bloodshed, although the Syrian economy is suffering.

EU experts will work out later exactly which goods will be included in the new embargo.

The anti-Assad uprising began in March 2011 with largely peaceful protests calling for his ouster. The regime responded with a withering military crackdown, which has prompted may in the opposition to take up arms against the troops.

The Syrian government blames the uprising on terrorists acting out a foreign conspiracy. It said Monday that “terrorists” had killed a doctor in southern Syria, two military officers in Hama and two others in the south.

World powers remain divided on how to resolve the crisis. The U.S. and other Western nations have called on Assad to step down, but say they will not intervene militarily. Russia and China have stood by Damascus, twice protecting it from censure by the U.N. Security Council.

All, however, have backed Annan’s plan, which does not specify that Assad must leave power. Most Syria analysts doubt it will end the conflict.