Countries from Australia to the UK have expelled Syrian diplomats in a coordinated move to step up pressure against the regime.
Syrian troops have shelled the central city of Homs in an apparent violation of an internationally brokered ceasefire, activists said.
Five protesters killed in Syria and UN envoys meet to vote on a resolution authorising the deployment of 30 UN observers to the country.
China has said that it is considering sending observers to monitor a week-old truce in Syria, that has so far failed to put an end to a year of bloodshed.
China is "seriously studying" the idea, Foreign Ministry spokesman Liu Weimin told a daily news briefing.
United Nations Secretary-General Ban Ki-moon has said he is not "underestimating the gravity of the situation" in Syria, but believes there is an opportunity for progress and recommended the Security Council approve a 300-strong UN observer mission.
Mr Moon said in a letter to the council that he will consider developments on the ground, including the consolidation of the ceasefire, before deciding when to deploy the expanded mission, which is larger than the 250 observers initially envisaged.
– Ban Ki-moon, UN Secretary General
Violent incidents and reports of casualties have escalated again in recent days, with reports of shelling of civilian areas and abuses by Government forces. The Government reports violent actions by armed groups. The cessation of armed violence is therefore clearly incomplete.
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.