Syria is still witnessing unacceptable levels of violence despite a ceasefire in place since 12 April, diplomats have quoted envoy Kofi Annan as telling the UN Security Council.
Briefing the Council in a closed-door meeting, Mr Annan said he was particularly concerned about violence surging after UN monitors leave cities.
Activists said 70 people were killed on Monday, mostly in the city of Hama.
The UN wants to increase its monitors in Syria from a handful to 300.
Mr Annan said the overall situation was "entirely contrary to the will of the international community".
He said that reported government firing on protesters in Hama, if confirmed, was "reprehensible and unacceptable".
He also called for the rapid deployment of the observer mission.
The BBC's Laura Trevelyan at the UN says the US and its European allies are likely to question Mr Annan on how the shaky ceasefire can be
The Security Council remains divided on Syria, with the Western powers pushing for tougher action while Damascus's allies, Russia and China, say Mr Annan's peace plan is sufficient.
Earlier Mr Annan's spokesman, Ahmed Fawzi, said in remarks broadcast on UN television that satellite imagery showed the Syrian government was failing to withdraw heavy weapons from urban centres.
He also said there were credible reports that people who met monitors were then approached and sometimes killed by security forces.
Earlier an activist in Hama told the Associated Press news agency 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 northern Arbaeen and Mashaa al-Arbaeen districts on Monday. Some 40 people were said to have died.
Another activist, named as Samer, told the BBC there have been government attacks every day and the regime is constantly in breach of the ceasefire.
"What happens is the observers visit neighbourhoods in the city, then once they leave, the shooting and shelling starts again," he said.
The government said security forces "pursued armed terrorist groups" which had been attacking and killing citizens in the area.
Syrian TV also reported a car bomb had exploded in the centre of the capital on Tuesday, injuring three people.
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.