Government forces followed the pattern established since the cease-fire, resuming attacks where the United Nations had just visited, while soldiers remained largely quiet in the places where the unarmed monitors were walking around. The Damascus suburb of Douma, which staged a massive antigovernment protest when the observers visited Monday, was shelled heavily on Wednesday morning, activists said.

Only about a dozen monitors have deployed in the country so far, but Syrians have already soured on the experience, blaming the monitors for being powerless in the face of further violent attacks despite the cease-fire technically in effect since April 12. The United Nations Security Council continues to back the peace plan, however, with the full contingent of 300 inspectors expected to deploy over the next couple months. Susan E. Rice, the American envoy to the United Nations, said late Tuesday that the presence of the monitors had diminished the violence and the point was to get them all deployed as quickly as possible. She quoted Hervé Ladsous, the head of all United Nations peacekeeping operations, as telling the council that 30 observers would be in place by the end of April, and 100 within a month.

One of the issues is that Syria has rejected one monitor on the basis of his nationality and informed the United Nations that it will not accept any monitors from countries among the “Friends of Syria,” an informal grouping of some 70 countries that has supported the opposition in its quest for political change.

“The onus remains on the Syrian government to halt the violence,” Ms. Rice said, “then subsequently on both sides to maintain a cessation of violence.” Kofi Annan, the joint United Nations and Arab League envoy who negotiated the truce and who also briefed the Security Council, said that Syrian Army attacks after the monitors departed on people who staged antigovernment protests while they were there is “unacceptable and reprehensible, if true.”

Mr. Annan told the council that he had received a letter on Saturday from the Syrian foreign minister saying troops and heavy equipment had been withdrawn from populated areas, with the police now in control of security. Daily videos of tanks and armored personnel carriers patrolling and even firing in populated areas openly contradict what was contained in the letter, although Mr. Annan did not phrase it that starkly.

“The only promises that count are the promises that are kept,” Ms. Rice quoted him as saying. Ms. Rice said several council members “expressed their skepticism” about the truth of what the letter from the Syrian Foreign Ministry said. In Geneva, the spokesman for Kofi Annan, the United Nations and Arab League envoy who negotiated the plan, said on Tuesday he was aware that Syrians were being targeted after the observers passed through and that attacks recommenced.

“We have credible reports that when they leave, the exchanges start again, that these people who approach the observers may be approached by the Syrian security forces or the Syrian Army or even worse, perhaps killed, and this is totally unacceptable,” Ahmad Fawzi, Mr. Annan’s spokesman, told United Nations television.

Satellite images and other reports showed that Syria had not fulfilled its obligation under the six-point peace plan. Contrary to the agreement, Syria still deployed heavy weapons in urban centers, Mr. Fawzi said. His remarks were similar to the points Mr. Annan made in his closed briefing before the Security Council later on Tuesday.

“We are calling on the Syrian government to fully implement its commitments under the cease-fire,” Mr. Fawzi said.

He noted that cities like Homs and Hama that had been embattled for months enjoyed at least a temporary quiet while the observers were present. Two were now stationed in both cities, but he conceded that more were needed.

“With 11 or 12 monitors, you can’t be everywhere, and there are many cities that have seen destruction and have seen fighting, and we have to be present,” Mr. Fawzi said. “With up to 300, we will be able to monitor more cities than two or three at a time.”

The inspectors returned to the central city of Hama on Tuesday — but the mood had soured. Their first visit, on Sunday, prompted a huge antigovernment demonstration. On Monday, however, government forces shelled and raided two poor, mostly Sunni Muslim neighborhoods that had protested, killing around 30 people, according to activists.

“The observers were received in a very different way today,” said Manhal, an activist reached via Skype, who used only one name out of fear of retribution. “Anger and sorrow surrounds Hama, and they are the reason behind the killing,” he said. “People know if they meet them they will either be killed or arrested. I have lost faith in these visits.”

A far smaller group of people turned out this time to talk to the observers, videos posted on YouTube indicated. Some people in the Arbaeen neighborhood covered their faces with scarves when approaching the monitors.

But some still held out hope that the monitors might help. One woman swathed in black was shown pleading with Col. Ahmed Himmiche, the Moroccan commander of the advance team. “We are being slaughtered!” she yelled, her hands shaking in front of her face. “Our children are gone! Burning and killing and slaughter! For God’s sake, protect us, if you really came as observers for us!”

In northern Idlib Province, where the monitors have yet to visit, protesters in the town of Binnish used sarcasm to convey their message. In the midst of an antigovernment protest, a small group of students, dressed like observers in eggshell-blue berets and vests, wandered through the crowd. They were wearing sunglasses and tapping walking sticks, as if they were blind, and had toilet paper stuffed in their ears. “There is nothing new on the ground,” said one of the students in the video, shown on Al Jazeera.

In Damascus, an intelligence officer was shot dead in Barzeh, a northern suburb, according to a statement from the Syrian Observatory for Human Rights, based in Britain. The man was suspected of identifying local protesters for arrest. A retired lieutenant colonel and his brother, also an officer, were assassinated on the outskirts of Damascus as well, reported SANA, the state-run news agency, which accused “terrorists” of the attack.

The official media also said that “terrorists,” their generic label for any government opposition, planted explosives that went off under the vehicle outside the Iranian Culture Center. Pictures showed the cab of the unmarked Mazda pickup truck used by the security services filled with shattered glass and blood, but no destruction to the buildings around it.

In the meantime, the renewed campaign to detain peaceful activists continued unabated. Salameh Keleh, a Palestinian-Syrian writer outspoken in his criticism of the government, was arrested Tuesday, according to the Activists News Association.

Syria has barred most independent media, making verification of claims difficult.