A sharp rise in the number of deaths in the Syrian uprising is casting fresh doubt on the success of a UN peace plan, and testing a ceasefire deal.
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.
One activist there, Mousab Alhamadee, told Associated Press news agency that dissidents were punished for coming out to greet the visiting UN observers on Sunday, when they chanted "Long live Syria! Down with Assad".
Syrian troops reportedly fired shells and automatic weapons in the Musha al-Arbeen district on Monday. Some 40 people were said to have died.
Another activist told AP: "Those observers brought destruction upon us. Any area they visit, the regime attacks. It's a tragedy."
The government's brief account said only that security forces "pursued armed terrorist groups" which had been attacking and killing citizens in the area.
A similar situation was reported in the city of Deraa, where anti-government protesters who greeted UN observers were later reportedly subjected to tear gas and gunfire from security forces.
The BBC's Jim Muir, in neighbouring Beirut, says that, in contrast, the situation in the flashpoint city of Homs has been quieter, as two UN observers are now permanently stationed there.
He says that is why UN officials are calling this a "pivotal moment" in the pacification process, hoping that the deployment of the full contingent of 300 monitors over the coming weeks will change the political dynamics and prepare the way for dialogue towards a settlement.
The UN currently has 11 unarmed observers under its Supervision Mission to Syria (UNSMIS). The UN passed a resolution on 14 April for a total of 30, then another resolution last Saturday to increase the number to 300.
Mr Annan, the UN and Arab League envoy to Syria, has said he believes the deployment of the 300 monitors can begin next week
But following Monday's violence, US Ambassador to the UN Susan Rice told the Security Council: "The regime's long track record is one of dependable deceit and deception, thus this Syrian mission is unusually risky and dangerous."
UN political chief B Lynn Pascoe added that "human rights violations are still perpetrated with impunity".
Mr Annan will brief the Security Council for the third time at about 19:00 GMT on Tuesday, speaking to the closed meeting by video-conference.
On Monday, UN Secretary General Ban Ki-moon again insisted that Syria "should provide full protection of our monitors and ensure their freedom of access, freedom of movement".
The UN says about 9,000 people have died since pro-democracy protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.