President Barack Obama has described the Syrian army's mortar and rocket attacks in the city of Homs, as "outrageous bloodshed".
One activist network said 110 people were killed in Homs on Thursday as the government continued a push aimed at crushing rebel forces.
Hundreds have been reported killed in the offensive over the past week.
Germany has meanwhile welcomed the possibility of the return of the Arab League mission to the country.
The international community is struggling to find a way to resolve the crisis after Russia and China blocked a UN resolution drafted by Arab and European countries on Saturday calling on President Bashar al-Assad to hand over power to his deputy.
EU foreign affairs chief Catherine Ashton called on Russia to "recognise the reality of the situation on the ground".
Restarting the Arab League mission in partnership with the UN "would serve peace and balance," German Foreign Minister Guido Westerwelle said, according to AFP.
The mission "would be a clear signal by the international community to the Assad regime," as would the appointment of a UN special representative, he added.
Continue reading the main storyThe BBC's Paul Wood reports from inside Homs
Brutal 'civil war'
Shelling of Homs
Despair in Homs
Picking up bodies
UN Secretary-General Ban Ki-moon had earlier said that his counterpart at the Arab League Nabil El-Araby had proposed sending a mission to Syria with UN help.
Mr Ban condemned the "appalling brutality" of the military operation in Homs.
Meanwhile, the US State Department said that efforts were continuing to put together a "friends of Syria" group of countries.
Assistant Secretary of State for Near East Affairs Jeffrey Feltman has been dispatched to Morocco, France and Bahrain to discuss the group's membership and its mandate, AP reports.
'Hospitals overwhelmed'
Homs, Syria's third-largest city, has been a leading focus of unrest in the 11-month uprising against President Bashar al-Assad's rule.
The assault on Homs is focused on districts that are controlled by rebel forces, in particular the Baba Amr district.
The BBC's Jim Muir, in neighbouring Lebanon, says there are armed rebels in the area, but also many civilians.
During lulls in the onslaught, Syrians are using loudhailers to appeal for blood donations and medical supplies, the Associated Press news agency reports.
The Human Rights Watch group said the blockade of the city by government forces meant victims of the shelling were being denied adequate medical treatment, saying makeshift hospitals were being overwhelmed with the dead and wounded.
The army says it is fighting foreign-backed armed groups. Army defectors have joined rebel forces in Homs and other parts of Syria in recent months.
Government troops have stepped up operations at Syria's borders in an effort to stop arms reaching the rebels, reports say.
Continuing army operations against rebels are also being reported in the northern city of Idlib, the southern province of Deraa, and the town of Zabadani, north of Damascus.
The Local Co-ordination Committees, a network of anti-government activists monitoring the violence, say a total of 137 people were killed around the country on Thursday, including 110 in Homs.
Syria restricts access to foreign media and casualty figures cannot be independently verified.
'Tightening stranglehold'
British Foreign Secretary William Hague said efforts were focused on tightening the "diplomatic and economic stranglehold on this murdering regime", but said there were no plans for Britain to arm Syrian rebels.
And in a blog post the British ambassador to Syria Simon Collis - who has been recalled to London for consultations - said "time and again" he had witnessed peacefully protesting Syrian civilians being beaten by "regime thugs".
Germany had earlier announced it was expelling four diplomats from the Syrian embassy in Berlin, following the recent arrest of two people suspected of spying on Syrian opposition figures in Germany.
Libya said it was giving Syria's charge d'affaires and staff in Tripoli 72 hours to leave the country, months after it recognised the Syrian opposition as the legitimate authority.
Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began last March.
The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm.
Mr Assad's government says at least 2,000 members of the security forces have been killed.
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