International envoy Kofi Annan has described the situation in Syria as "bleak" during a closed briefing to the UN Security Council.
Annan said he was alarmed at reports that government troops entered the central city of Hama, firing automatic weapons and killing a number of people.
He echoed UN Secretary-General Ban Ki-moon in calling the current situation "unacceptable" and called on the Syrian government to immediately implement his six-point peace-plan.
Annan urged Damascus and the opposition to respect the April 12 cease-fire.
After the briefing by Annan and UN Undersecretary-General for Peacekeeping Herve Ladsous, the U.S. ambassador, Susan Rice, said Syria was putting limits on the deployment of cease-fire monitors.
"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 UNSMIS [UN Supervision Mission in Syria] staff members from any nations that are members of the Friends of Democratic Syria. He underscored that, from the UN's point of view, this is entirely unacceptable," Rice told reporters at UN headquarters in New York.
The 14-nation Friends of Syria includes the United States, Britain, France, Saudi Arabia, Turkey, and Qatar.
All of those countries have said that Syrian President Bashar al-Assad has lost his legitimacy because of his 13-month assault on pro-democracy protesters.
Rice also said Ladsous had confirmed to the Security Council that it will be another month before 100 of the anticipated 300 observers reach Syria to help monitor the country's shaky cease-fire.
Rice repeated the U.S. position that the onus is on Assad's regime to halt the violence.
"I will speak for the United States, as I have been saying over the course of the last several days -- our patience is exhausted," Rice said.
"The fact that the violence continues despite the so-called cease-fire of April 12 is, in our view, not only unacceptable but reprehensible, and we've been very clear in supporting the dispatch of the balance of the monitoring mission that the onus remains on the Syrian government to halt the violence and then subsequently on both sides to maintain a cessation of violence and allow the observers to move freely and do their work without any obstruction," Rice added.
In the latest violence, three Syrian military officers were killed in Damascus, state media and opposition groups said, and at least three people were wounded in a car bomb blast in the capital.
Based on AP and Reuters reporting