In his letter to the UN Security Council, Secretary-General Ban Ki-Moon says a "clear signal" must be sent to Syria about the need for a total ceasefire.
The UN Secretary General Ban Ki Moon says he wants to send more monitors to Syria to observe the fragile ceasefire. An advance team is already there, but the UN wants to expand it to 300 members.
In a letter to the Security Council the UN Secretary General Ban Ki-Moon says Syria has not complied with an obligation to withdraw troops and heavy weapons from towns.
Sporadic violence across Syria casts doubt over UN plans to expand a team of observers to the country, US envoy to the UN Susan Rice said.
The US Ambassador said the continued violence, in violation of a ceasefire, was "gravely concerning".
"Should the violence persist and the ceasefire, or cessation of violence more aptly, not hold, that ... will call into question the wisdom and the viability of sending in the full monitoring presence."
Videos purporting to show shelling in Homs earlier today have been posted on social media websites on the same day as UN observers arrive in Syria to monitor the ceasefire. The videos claim to show the neighbourhoods of Juret al Shayah and al-Karabees.
ITV News cannot independently verify the veracity of the footage or the date it was shot.
Activists say Syrian troops are shelling neighbourhoods in the opposition stronghold of Homs, hours after the arrival of six UN observers in Damascus.
The Local Co-ordination Committees and the British-based Syrian Observatory for Human Rights also said two people were killed in Hama in central Syria today when security forces opened fire on their car.
Five UN ceasefire monitors have arrived in the Syrian capital Damascus, more observers are expected to enter the country on Monday.
Syria said it would stop what it called "terrorist groups" from continuing criminal acts, state TV quoted a security source as saying, casting further doubt on whether the ceasefire would hold.
The Syrian state news agency SANA said:
(Security forces), based on their duty to protect civilians and the country, will stop terrorist groups from continuing their criminals acts and the killing of civilians.
Since the announcement of an end to military operations, terrorist attacks have increased by dozens, causing a large loss of life.
The UN's Ban Ki-moon urges "in strongest possible terms" to maintain the ceasefire in Syria.
The Syrian Government has said that it cannot be responsible for the safety of UN monitors unless it is involved in all steps on the ground and confirmed the number of monitors to rise to 250 as agreed.
It sas also stressed that it has the right to respond to any acts of aggression by armed groups.