Activists said nearly 70 people were killed on Monday, most in a government crackdown in the city of Hama.
The US said the UN monitoring mission was “risky and dangerous”.
The UN is pressing to increase its monitors from a handful to 300 and chief envoy Kofi Annan is due to brief the Security Council again on Tuesday.
A UN-brokered ceasefire came into effect on 12 April, but although the overall level of violence has dropped since then, President Bashar al-Assad’s government has been accused of failing to abide by key terms of the truce plan, including pulling all forces from urban areas and allowing peaceful demonstrations.
Activists have pointed to the situation in Hama as an example of the dangers of the UN mission.
Syrian troops reportedly fired shells and automatic weapons in the Musha al-Arbeen district on Monday. Some 40 people were said to have died.