The UN Security Council is set to vote on a draft resolution condemning Syria’s deadly crackdown, a day after reports of violence in the city of Homs that activists said left more than 200 people dead.
Negotiations between council members and co-member Russia were continuing on Saturday as Moscow, a strong ally of the Syrian government, signaled it would veto any call for President Bashar al-Assad's removal.
Even so, Western diplomats said they are determined to conduct the vote on the draft resolution backing an Arab League call for Assad to give up power.
British UN Ambassador Mark Lyall Grant also said: "We expect a vote today."
In remarks meant to coincide with the UN vote, US President Barack Obama accused the Syrian government of murdering civilians in an "unspeakable assault" in Homs, and demanded that Assad step down.
"Assad must halt his campaign of killing and crimes against his own people now. He must step aside and allow a democratic transition to proceed immediately," Obama said in a statement.
The diplomatic developments come as activists said the Syrian army has carried out an assault on Homs’ neighbourhood of Khaldiyeh on Friday night and continued overnight.
The Local Co-ordination Committees activists network said that more than 500 people were injured after the army used tanks, mortars and machine guns in the assault on the opposition stronghold in the central city.
Al Jazeera's Mysa Khalaf, reporting from Beirut in neighbouring Lebanon, said sources in Syria told her the bombardment of the area started after the opposition Free Syrian Army, a group of army defectors, attacked Syrian army checkpoints and killed about 10 soldiers.
"I've been told that the main public hospital is completely overwhelmed and people have set up makeshift clinics in mosques. They are running low on supplies of blood," she said. "Several buildings have been destroyed."
The Syrian government denied the assault, saying the reports were part of a 'hysterical campaign'' of incitement by armed groups against Syria.
The government said the recent reports of violence were meant to sway the UN Security Council vote.
Sergei Lavrov, Russian foreign minister, said on Saturday that if a resolution is put to a vote without taking Moscow's opinion into account, it will only lead to "another scandal" at the Security Council.
Lavrov said Moscow had objections to what he termed "The imposition of the terms and conditions of the dialogue, which must be started without prejudging the results".
He also said that "measures must be taken to influence not only the government ... but also the armed groups, because unless you do it both ways, you are taking sides in a civil war".
Moscow has been a strategic ally of Syria through its decades under Assad dynastic rule and a major arms supplier to Damascus, and so bristles at outsiders trying to dictate internal political change in Damascus.
Rafeeq Abdel Salaam, Tunisia's foreign minister, announced on Saturday that his government started the procedure for expelling the Syrian ambassador from Tunis.
Earlier on Saturday, British police used batons and riot shields to hold back protesters trying to storm the Syrian embassy in London for the second time in one day.
Police brought in sandbags and riot gear to regain control of the surging crowd, which lobbed objects at the embassy, situated near Buckingham Palace.
Demonstrators angry with Assad stormed five Syrian embassies in Europe and the Middle East, including in Cairo.
Speaking to Al Jazeera from Homs, activist Hadi al-Abdallah said government forces were targeting the neighbourhoods of Bab Tadmour, Bab Dreib, and Karm el-Shami simultaneously, as the military campaign in Khaldiyeh intensified.
Al Jazeera meets activists in Homs who are defying bullets to document violence
"There has been non-stop bombardment in Bab Amr [neighbourhood of Homs] ... They've been bombarding Bab Amr and Khaldiyeh non-stop with mortar bombs and tank shells ... it's just random bombarding on rooftops," Danny Abdul Dayem, an activist, told Al Jazeera early on Saturday.
The UK-based Syrian Observatory for Human Rights said 14 soldiers were killed in clashes with opposition fighters and that five army defectors had lost their lives.
The group cited witnesses saying 217 people had been killed in Homs, 138 of them in Khaldiyeh.
The opposition Syrian National Council decried Saturday's violence as a "horrific massacre".
"The Syrian National Council calls on everyone around the world to speak up and do something to stop the bloodshed of innocent Syrians," it said in a statement.
The country has witnessed the deaths of more than 5,400 people according to UN figures, since anti-Assad protests errupted in March 2011.