UN-Arab League special envoy Kofi Annan told the UN Security Council on Tuesday that the situation in Syria is "bleak."
He also expressed alarm at media reports that Syrian troops continue to carry out military operations in towns where UN observers are not present.
"If confirmed, this is totally unacceptable and reprehensible," Annan said.
He was referring to 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.
Human rights activists in the city told reporters than 30 people were killed.
Annan echoed UN Secretary-General Ban Ki-moon, who called the current situation "unacceptable."
Annan 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.
Annan said the minister's letter is "encouraging" and would make "a real difference ... if it is scrupulously applied." He added, "It should be understood that the only promises that count are the promises that are kept."
Annan also urged the Security Council to accelerate the deployment of the 300-strong UN observer force authorized by the council, saying it was "crucial" to verify what is happening and "change the political dynamics."
Earlier, Annan's spokesman Ahmad Fawzi, said that satellite imagery and other credible reports show – despite Assad's claims to the contrary – that Syria's military is continuing its now 14-month crackdown.
Fawzi also cited credible reports also indicate that "people who approach the observers may be approached by security forces or Syrian army and harassed or arrested or even worse, perhaps killed."
US Ambassador Susan Rice told reporters after the briefing that "several council members expressed their skepticism 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country is Syria's most important ally, noted that some council members said "they have information" that Syria has not withdrawn its troops and heavy weapons.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," Churkin told reporters.
He added, "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 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."
At least 9,100 civilians have been killed Assad's brutal repression of anti-regime protests, which has turned Syria's so-called Arab Spring revolt the bloodiest and most entrenched thus far.