U.N. sources: Syria won't accept observers from certain countries U.N. official: Syria says it will not accept monitors from countries aligned with 'Friends of Democratic Syria' Report: 13 U.N. monitors are on the ground in Syria Hillary Clinton: It is "absolutely deplorable" if the regime is killing residents who spoke to U.N. monitors At least 38 people were killed across Syria on Tuesday, opposition activists say (CNN) -- U.N.-Arab League special envoy Kofi Annan has expressed dismay over reports that the Syrian government resumes brutal attacks on cities once U.N. monitors leave -- the latest sign that diplomatic efforts are failing to quash the bloodshed in the country. "I am particularly alarmed by reports that government troops entered Hama (Monday) after observers departed, firing automatic weapons and killing a significant number of people," Kofi Annan told U.N. Security Council members Tuesday. If confirmed, this is totally unacceptable and reprehensible. Annan said Syria's foreign minister told him heavy weapons and troops had been withdrawn from population centers and that military operations had ended -- in line with a six-point peace plan laid out by Annan and accepted by the Syrian regime. Annan described the communication as "encouraging," but added, "the only promises that count are the promises that are kept." Opposition activists said escalating carnage -- including at least 38 deaths on Tuesday -- is proof that Damascus keeps breaking its promises. Annan spokesman Ahmad Fawzi corroborated that view. He said the Syrian regime claims it has pulled back heavy armor, yet "satellite imagery, however, and credible reports show that this has not fully happened." A handful of U.N. observers have visited besieged cities such as Hama, Douma and Homs -- a bastion of anti-government sentiment -- in recent days. But the government doesn't just renew attacks on cities after the observers leave -- they also target residents who spoke out to U.N. monitors, opposition activists say. "They are all the time watched by the security forces of the regime," said one opposition activist, who is not being identified for safety reasons. U.S. Secretary of State Hillary Clinton expressed concern over claims that the Syrian government was killing residents who speak out. "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 U.N. Security Council recently authorized sending up to 300 monitors for 90 days. They are tasked with observing a cease-fire that was supposed to begin April 12. Two Chinese nationals are among the latest U.N. observers to enter Syria, China's state-run Xinhua news agency said Tuesday, bringing the number of monitors on the ground to 13. Thirty observers are expected in the country by Monday, and 100 within a month, said U.S. Ambassador to the U.N. Susan Rice, citing comments by Herve Ladsous of U.N.'s Department of Peacekeeping Operations. "Mr. Ladsous reported that the Syrian government has refused at least one observer based on his nationality, and that Syrian authorities have stated they will not accept (monitoring mission) staff members from any nations that are members of the 'Friends of Democratic Syria,'" Rice said Tuesday. He underscored that from the U.N. point of view this is entirely unacceptable. Dozens of countries and international groups have met in "Friends of Syria" conferences in recent months to try to help stop the slaughter in Syria. CNN cannot independently verify reports of violence and deaths within Syria, as the government has restricted access by international media. The country has been engulfed in violence since March 2011, when the government started cracking down on demonstrators who were peacefully protesting al-Assad's regime. Opposition members say they want democratic reforms and the ouster of al-Assad, whose 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. CNN's Joe Vaccarello, Jill Dougherty and Amir Ahmed contributed to this report.