(CNN) -- The Syrian military is halting violence in areas entered by U.N. observers but resumes attacking once the monitors leave, the U.N.-Arab League joint special envoy said Tuesday.
"I am particularly alarmed by reports that government troops entered Hama yesterday after observers departed, firing automatic weapons and killing a significant number of people," Kofi Annan told Security Council members.
"If confirmed, this is totally unacceptable and reprehensible."
CNN obtained a transcript of Annan's remarks from Security Council diplomats.
Annan added that Syria's foreign minister told him heavy weapons and troops had been withdrawn from population centers and military operations had ended -- as called for by a six-point peace plan laid out by Annan and accepted by the Syrian government.
"The police and the forces of law and order, which will exercise self-restraint" were maintaining law and order, he said he was told.
Annan described the communication as "encouraging," but added, "The only promises that count are the promises that are kept."
Opposition activists cited continuing carnage, including at least 35 killings on Tuesday, as evidence that the promises from Damascus were not being kept.
That view was corroborated by Annan spokesman Ahmad Fawzi. "We are calling upon the Syrian government to fully implement its commitments under the six-point plan, and this means withdrawal of heavy armor from population centers, and back to the barracks," he said. Syria is "claiming that this has happened," he added, "satellite imagery, however, and credible reports show that this has not fully happened."
In Washington, U.S. Secretary of State Hillary Clinton expressed dismay over the report that the Syrian government was killing members of the opposition who have spoken with U.N. monitors.
"It is absolutely deplorable if there is this kind of intimidation and harassment and possible violence against those Syrians who have every right to meet with and discuss the situation with the monitors," she said.
Clinton said U.S. officials were preparing "additional steps" in case the violence continues or if the monitors are prevented from doing their work.
The council recently authorized sending up to 300 monitors for 90 days. They are tasked with observing a cease-fire that was to have begun April 12.
"Taken as a whole, the level of violence has decreased" since that date, Annan said. "This, however, does not cover the spike in violence reported yesterday."
The opposition Local Coordination Committees of Syria reported 80 killings on Monday across the nation, with at least 50 of them occurring in Hama.
Since the cease-fire began, demonstrations have increased "and there have been unconfirmed reports of the use of live fire by troops on some occasions," Annan added.
He called for the full complement of observers to be deployed quickly.
Thirty observers are expected to be in the country by April 30 and 100 within a month, said U.S. Ambassador Susan Rice, who cited remarks made by Herve Ladsous, the under-secretary-general for the U.N.'s Department of Peacekeeping Operations.
An explosion rocked central Damascus on Tuesday, according to the opposition Syrian Observatory for Human Rights. The group had no immediate information about possible casualties.
Syria said a bomb was placed under a car and wounded the driver when it detonated in the neighborhood of al-Marjeh, the state-run news agency SANA said.
On the opposition side, of the 35 people killed Tuesday, three died as a result of torture, according to the opposition Local Coordination Committees of Syria.
Thirteen of the deaths occurred in the beleaguered city of Homs, 11 in the Damascus suburbs, three in Damascus, two in Hama, three in Idlib, one in Aleppo, one in Daraa and one in Bokamal, the group said.
Regime forces pummeled Homs and Hama with heavy shelling days after U.N. observers left those cities, the LCC said.
Tank artillery and mortar rounds rained on the city of Douma on Tuesday, according to an opposition activist identified only as Fateh for safety reasons.
In Idlib, security authorities moved 20 bodies from a hospital to an undisclosed location, a move that coincided with the arrival of U.N. observers in the area, the LCC said.
In the Damascus suburb of Douma, an explosion was reported, the group said. In another suburb, Karafbatna, several activists were reported to have been arrested, apparently at random, at a checkpoint in the center of the city, the group said.
In Damascus, a government intelligence officer was killed Tuesday morning, the Syrian Observatory for Human Rights said.
Syria said armed terrorists killed a retired officer and his brother in the province of Damascus on Monday night, in the neighborhood of Fadel.
A SANA report also said the "bodies of seven army and law enforcement martyrs" were buried Tuesday.
International leaders, including U.S. President Barack Obama, are ratcheting up pressure on the Syrian regime of Bashar al-Assad.
"I have signed an executive order that authorizes new sanctions against the Syrian government and Iran and those who abet them for using technologies to monitor and track target citizens for violence," Obama said Monday. "It's one more step that we can take toward the day that we know will come: the end of the Assad regime."
In addition, European Union foreign ministers agreed to ban the export to Syria of goods and technology that might be used by Damascus to produce chemical or biological weapons.
The E.U. also banned the export of luxury goods to Syria, according to a statement Monday by British Foreign Secretary William Hague.
"Despite the urgent need for Assad to end the violence immediately, he and his close supporters continue to lead comfortable lives," Hague said.
Though they agreed to beef up the observer mission, Russia and China, two permanent countries on the 15-member Security Council, have vetoed attempts to take tougher action against the Syrian regime.
Russian U.N. Ambassador Vitaly Churkin defended his country's position.
"As a matter of principle, we believe that the U.N. Security Council is not about regime change," Churkin told CNN's Christiane Amanpour on Monday. "We believe that ... if there is crisis in a country, the role of the international community should be to help the parties involved to find a political, peaceful way out of this crisis.
"And when we saw some of the resolutions -- which included sanctions -- we knew that those were resolutions which were heading in the direction of regime change by force, which would, in turn, lead only to much more bloodshed in Syria."
The Annan peace plan calls for the government and the opposition to end the violence, provide access for humanitarian groups, release detainees and start a political dialogue.
CNN cannot independently verify reports of violence and deaths within Syria, as the government has restricted access by international media.
Syria has been engulfed in violence since March 2011, when the government started started cracking down on demonstrators who were peacefully protesting al-Assad's regime. The president's family has ruled Syria for 42 years.
The United Nations estimates at least 9,000 people have died since the protests began, while activist groups put the death toll at more than 11,000. Susan Rice, U.S. ambassador to the United Nations, put the total at 10,000.
CNN's Jill Dougherty, Amir Ahmed, Joe Vaccarello, Saad Abedine, Holly Yan and Salma Abdelaziz contributed to this report.