Syrian forces on Monday shelled rebel bastions in Homs and Damascus despite pleas for help from the opposition and a warning by the United Nations that such bombardment amounted to crimes against humanity.
US President Barack Obama, meanwhile, is due to meet his Russian counterpart Vladimir Putin at a G20 summit to discuss differences over what to do about the bloody conflict.
"Shelling and shooting renewed in Homs city, with explosions heard in the Khaldiyeh neighbourhood," the Britain-based Syrian Observatory for Human Rights said, reporting that at least 19 people were killed nationwide on Monday.
The watchdog had on Sunday reported regime shelling of the Old City of Homs and outlying towns including Rastan and Talbisseh in a bid to crush armed insurgents, adding that about 1,000 families were stuck in the area.
The situation in Rastan was particularly dire and most residents had fled to neighbouring villages, anti-regime activist Nidal al-Hakim told AFP via Skype on Sunday.
Many people were "critically" wounded and there was a shortage of medication, while the authorities had severed water and electricity supplies to the besieged city, Hakim said.
A YouTube video uploaded on Sunday showed clouds of black and grey smoke billowing over buildings in the Old City of Homs.
The thump and rattle of shelling and gunfire ripped through the silence of what appeared to be a ghost town.
Another video showed widespread destruction and deserted streets in the city's Jourat al-Shiah district.
"We don't have any milk for the children, nor water, nor electricity," a mother of two whose house was destroyed said on the video.
"We are not scared. We don't want weapons or money. We just want a way to get our children out of here."
The authenticity of the videos could not immediately be verified.
Housing several rebel strongholds, Homs has been under intermittent attack by the regime ever since its district of Baba Amr was pounded relentlessly for a month before being retaken by government forces in early March.
UN rights chief Navi Pillay on Monday demanded the immediate cessation of such bombardments of populated areas, warning that such violence amounted to crimes against humanity.
"The government of Syria should immediately cease the use of heavy armaments and shelling of populated areas, as such actions amount to crimes against humanity and possible war crimes," she told the Human Rights Council in Geneva.
Pillay also called for a probe into an attack on a UN convoy a week ago in the town of Al-Haffe in the northwestern province of Latakia.
She said those responsible for such attacks on UN observers must be brought to justice.
The Observatory said that four people, including a rebel commander, were killed in an explosion in Mohassan town in the eastern province of Deir Ezzor.
It said clashes and shelling continued through the night in several areas of Damascus province, including the towns of Douma and Qoudsaya which have been under regime bombardment for the past five days.
The Local Coordination Committees (LCC), grouping opposition activists on the ground, said Qoudsaya came under "heavy shelling" and that "snipers are shooting at any moving object."
An activist added that "huge military reinforcements" had arrived there and that the wounded could not be treated because the shelling and sniper fire was so intense.
Overnight, regime forces backed by aircraft also pounded for more than seven hours a region known as the Kurdish Mountain in Latakia province, forcing many residents to flee, the Observatory said.
The escalation in violence prompted UN observers at the weekend to suspend their monitoring mission.
Mission chief Major General Robert Mood urged the warring parties to "allow women, children, the elderly and the injured to leave conflict zones, without any preconditions and ensure their safety."
Main opposition group the Syrian National Council on Sunday urged the United Nations to pressure the Syrian regime using Chapter VII of the UN charter, which allows measures to be imposed on a country under penalty of sanctions or force.
The opposition specifically demanded that observers tasked with monitoring a UN-backed ceasefire -- that has been flouted daily since going into effect on April 12 -- be armed.
UNSMIS, the UN Supervision Mission in Syria, on Saturday suspended its operations two months into its three-month mandate, blaming the intensifying violence threatening its 300-strong force.
Obama, meanwhile, is due to meet Putin later on Monday amid hopes that Russia, a strong ally of President Bashar al-Assad, will eventually help ease the crisis in Syria.
Russia also said it is preparing to send two amphibious assault ships to the Syrian port of Tartus where Moscow has a strategic naval base to ensure the safety of its citizens, the Interfax news agency reported.