Syria crisis: Shelling 'kills dozens' in restive Homs
The Syrian army has launched fresh mortar and rocket attacks in the city of Homs, as the government continues a push aimed at crushing rebel forces.
Activists say 95 people have been killed so far on Thursday. Hundreds have reportedly died since last week.
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.
UN chief Ban Ki-moon condemned the "appalling brutality" of the onslaught.
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.
The most important factor [in the resilience of the Syrian army] is the deepening and widening of the uprising over recent months. Those once wedded to peaceful protest now judge that taking up arms is the only viable option.
One important milestone has been the outbreak of violence this month in Syria's second city, Aleppo. The army is increasingly stretched across fronts that it did not have to worry about last year.
It took days to regain control of suburbs around Damascus, and the town of Zabadani - scarcely 20 miles (32km) north-west of Damascus - was entirely seized by rebels last month.
These growing commitments thin out, and increase the strain on, loyal units. The paradox is that more fighting means more defections, but failing to do so risks conceding territory on which the armed parts of the opposition can regroup and consolidate.
The assault on Homs, which began late on Friday, is focused on districts that are controlled by rebel forces.
The worst shelling has been in the Baba Amr district, where activists say 50 people were killed on Wednesday alone.
The BBC's Jim Muir, in neighbouring Lebanon, says there are armed rebels in the area, but also many civilians.
Ali Hazuri, a doctor in Baba Amr, told AFP news agency that the intense bombardment had resumed early on Thursday after an overnight lull.
"The shells are raining down on us and regime forces are using heavy artillery," he said.
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.
Syria restricts access to foreign media and casualty figures cannot be independently verified.
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 105 people have been killed around the country on Thursday, including 95 in Homs.
Meanwhile, Germany 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.
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".'Missed chance'
Speaking at the UN in New York on Wednesday, Mr Ban said the failure to agree a UN resolution on Syria had encouraged Damascus "to step up its war on its own people".Continue reading the main story
BBC correspondent Paul Wood's reports from inside Homs
He added: "I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighbourhoods, is a grim harbinger of worse to come."
He also said the Arab League was hoping to revive its monitoring mission in Syria - which collapsed last month amid the escalating violence - in possible collaboration with the UN.
British Foreign Secretary William Hague said efforts were focused on tightening the "diplomatic and economic stranglehold on this murdering regime", and said there were no plans for Britain to arm Syrian rebels.
The UN resolution vetoed by China and Russia backed an Arab League peace plan that would have seen President Assad hand power to a deputy to oversee a transition.
Russia said the proposal amounted to regime change.
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.