Fighting in Syria continues despite announcements from the Syrian government that it will comply with a UN-backed truce, the UN political affairs chief said.
The Syria government also said it has withdrawn troops and heavy weapons from cities and towns.
"The cessation of armed violence remains incomplete," said Lynn Pascoe UN undersecretary-general for political affairs.
He was speaking at the 15-nation Security Council during an open debate on the Middle East.
Mr Pascoe added: "Too many lives have been lost, human rights violations are still perpetrated with impunity.
"It is our hope that the deployment of observers will help to stop the killing and consolidate the calm."
More than 9,000 people have died in Syria during 13 months of fighting sparked by a popular uprising against President Bashar al-Assad.
At least 20 people were killed in shelling today, activists said.
Reports from Syria cannot be independently verified as the government has barred international journalists and rights groups.
The UN Security Council unanimously adopted a resolution on Saturday that authorised an initial deployment of up to 300 unarmed military observers to Syria for three months to monitor the truce implemented earlier this month.
But the resolution conditions the deployment of observers on a UN assessment of compliance with the truce.
This reflects European and US concerns that Syria’s failure to halt the violence, return troops to barracks and withdraw heavy weapons from towns makes the prospects for success slim.
Syrian Foreign Minister Walid al-Moualem wrote to UN-Arab League peace envoy Kofi Annan, who brokered the truce, on Saturday to inform him that Syria has now withdrawn heavy weapons and troops from Syrian cities.
Mr Annan is due to brief the Security Council tomorrow.
Mr Pascoe said: "It is essential that the government of Syria fully and immediately implement its obligations to stop using heavy weapons and to pull back military forces from populations centres.
"Measures taken by the Syrian authorities to date, including on the release of arbitrarily detained persons and the respect of the right to demonstrate peacefully, are clearly insufficient."
UN Secretary-General Ban Ki-moon will also brief the Security Council every 15 days about developments in Syria and submit to the Security Council proposals as needed for adjusting the mandate of the observer mission.
