Deadly air strikes pounded rebel-held towns across southern Syria on Wednesday, as relief groups sounded the alarm over a Russian-backed push for the region and its main city Daraa.
The violence has already ravaged civilian infrastructure in opposition towns, with three hospitals forced to shut down because of heavy raids.
As the humanitarian situation grows increasingly dire, aid organisations from the Red Cross to UNICEF called for more to be done to prevent civilian casualties.
The south is meant to be protected by a ceasefire put in place last year by Russia, Jordan, and the United States, but President Bashar al-Assad has set his sights on retaking the area.
After a week of air strikes and artillery fire on rebel towns across Daraa province, his troops turned to the opposition-held half of the provincial capital on Tuesday.
The bombing of rebel positions in the south continued into Wednesday, the Syrian Observatory for Human Rights said.
"There are rockets, barrel bombs and Russian and Syrian air strikes hitting rebel areas of Daraa, particularly the Daraa al-Balad neighbourhood," said Observatory head Rami Abdel Rahman.
Air strikes on the south killed 10 civilians on Wednesday including three infants, the Britain-based Observatory said, bringing the civilian death toll over the past week to 56.
"Horror knows no limit in Syria, where children are yet again caught in the crossfire of the latest wave of violence in the southwest," said Henrietta Fore, executive director of UN children's agency UNICEF.
"The children of Syria have lived through unacceptable suffering. This cannot become the new normal."
Rebels hold a horseshoe-shaped band of territory in the south, stretching across most of Daraa and the adjacent province of Quneitra.
Government forces have already isolated one end of it with the recapture of two strategic villages, but have yet to make any advance into the rebel sector of Daraa city.
As hostilities escalate, both the International Committee of the Red Cross and CARE called for restraint.
"Civilians are paying the price of another military offensive," said CARE's country director Wouter Schaap.
He warned that rights violations committed in other areas seized from rebels in recent years were being repeated.
"What we have seen in Aleppo, northern rural Homs, and Eastern Ghouta, is happening now in the south, where cities and towns are bombed daily, people are being uprooted and lack basic human necessities, such as water and shelter," said Schaap.
"We want to stress to all warring parties that international humanitarian law requires them to protect civilian infrastructure, in particular schools and hospitals," he added.
The assault's impact on health infrastructure has been particularly stark, with a total of five hospitals shuttered in the last week, said the Observatory.
Three of them were forced to shut down overnight from Tuesday to Wednesday, after heavy air strikes including by Russian warplanes on the rebel-held towns of Saida, Al-Mseifra and Al-Jiza, it said.
Syria has become infamous for attacks on health workers, with more facilities hit so far this year than in all of 2017.
The UN has warned that more than 750,000 lives were put at risk by the government's southern blitz.
More than 45,000 have already fled their homes, most of them to the sealed border with Jordan, it added.
Among them is Ahmad Abazeid, a media activist who fled the town of Al-Herak in the east of Daraa province for a nearby village that was facing less bombardment.
"People are lost -- they don't know where to go. Some are along the border with Jordan, others on the border with Israel," he told AFP.
"But the warplanes are following them wherever they go."
Jordan said late Tuesday that the border crossings would remain closed. Jordan already hosts more than 650,000 registered Syrian refugees and estimates the actual number is closer to 1.3 million.
The Norwegian Refugee Council appealed to Amman on Wednesday to grant asylum to those fleeing the latest violence.
"The fighting in Syria is squeezing people further and further south. They will eventually be left with nowhere else to turn," said the NRC's acting Regional Director Youri Saadallah.
"Jordan has done so much over the years to accommodate hundreds of thousands of refugees from Syria, but unfortunately the international community must rely on it to be generous once more."
In a bid to avoid greater bloodshed, Russia is leading talks with Syria, Jordan, Israel and the United States to reach a negotiated settlement for the south but so far there has been no public progress.