International envoy Kofi Annan has told the United Nations Security Council that the situation in Syria is "bleak" and expressed alarm at reports that government troops are still carrying out military operations in towns where UN observers are not present.
He expressed particular concern at media reports that government troops entered the central city of Hama on Monday after UN observers departed, firing automatic weapons and killing a significant number of people. Activists said more than 30 people were killed.
"If confirmed, this is totally unacceptable and reprehensible," Mr Annan said.
Annan echoed UN secretary general Ban Ki-moon, who called the current situation "unacceptable" and urged President Bashar Assad's government to immediately implement his six-point peace-plan, which would culminate with Syrian-led talks between the government and opposition aimed at reaching a peace settlement.
The joint UN-Arab League envoy said the speedy deployment of the 300-strong UN observer force authorised by the council on Saturday was "crucial" to verify what was happening on the ground and potentially "change the political dynamics".
The observer force also would provide the international community with "incontrovertible" information to increase pressure for a ceasefire by the government and opposition, he said.
Mr Annan briefed the security council by video conference hours after his spokesman, Ahmad Fawzi, told UN Television in Geneva that satellite imagery and other credible reports showed that, despite its claims, Syria had failed to withdraw all of its heavy weapons from populated areas as required by the ceasefire deal it accepted.
Mr Fawzi also cited credible reports that "people who approach the observers may be approached by security forces or Syrian army and harassed or arrested or even worse, perhaps killed".
Mr Annan did not mention either the satellite photos or the harassment and possible killing of people who talked to the observers in the text of his closed briefing, which was obtained by The Associated Press, but he stressed that "the government cannot cease action in one area to resume it in another".
He told the council the Syrian foreign minister had informed him in a letter on April 21 of the withdrawal of troops and heavy equipment from populated areas and the handover of responsibility to police for maintaining law and order.
He said he replied that this means troops should be back in barracks and weapons placed in storage "rather than operationally deployed" and that civilians should not be endangered by police actions.
Mr Annan said the minister's letter was "encouraging" and would make "a real difference ... if it is scrupulously applied". But he added pointedly: "It should be understood that the only promises that count are the promises that were kept."
US ambassador Susan Rice told reporters after the briefing that "several council members expressed their scepticism on the Syrian government's intentions and the veracity of statements contained in the Syrian foreign minister's letter".
Russia's UN ambassador Vitaly Churkin, whose country is Syria's most important ally, noted that some council members said "they have information" that Syria had not withdrawn its troops and heavy weapons.
"If this is the case, if the promise in the letter has not really been carried out, that would mean it is a breach of the promise they have made on Saturday," he told reporters. "I'm certainly going to bring it to the attention of Moscow that there is an issue that needs to be looked at."
US secretary of state Hillary Clinton told reporters in Washington that "the responsibility rests with Assad and with his supporters and his military to demonstrate a commitment to the Annan plan by silencing the guns".
"Unfortunately, the Assad regime has broken its commitments time and again," she said. "So even as we work to help deploy the monitors, we are preparing additional steps in case the violence continues or the monitors are prevented from doing their work."
Mr Annan said that in addition to the reported military attacks, Syria's implementation of the other points in his peace plan - including unrestricted access for journalists and humanitarian workers and allowing peaceful demonstrations - "remains partial".
He welcomed the council's initial authorisation of a 30-member advance team of UN observers, and its approval of a 300-strong UN observer team, stressing the importance of getting "eyes and ears on the ground" with the ability to move freely and swiftly.
Ms Rice said UN peacekeeping chief Herve Ladsous told the council that 11 observers were currently in Syria, including two in Homs and two in Hama. He predicted 30 observers would be on the ground by April 30 and 100 observers within a month, she said.
Mr Ladsous reported that the Syrian government refused at least one observer based on his nationality and stated that it would not accept any observers or civilians for the mission from countries that were members of the Friends of Democratic Syria, Ms Rice said. The group includes more than 70 countries including the US, many European countries and a number of Middle East nations.
"He underscored that from the UN's point of view, this is entirely unacceptable," Ms Rice said.