Firefights and shelling on Sunday shattered a weeks-old truce at the Yarmuk Palestinian camp in Damascus, but a ceasefire allowed residents to return to another besieged area near Syria's capital.
Syria's three-year conflict is estimated to have killed more than 140,000 people, forced millions to flee the homes, and taken a brutal toll on children increasingly threatened by disease.
On Sunday the United Nations said millions of children across the Middle East were to be vaccinated against polio after the crippling illness resurfaced in Syria for the first time in 15 years.
Meanwhile some good news emerged with Spanish journalist Marc Marginedas released by jihadists in Syria after six months in captivity, said his employer El Periodico newspaper.
The Syrian Observatory for Human Rights said the Al-Qaeda-linked Al-Nusra Front and the pro-regime Popular Front for the Liberation of Palestine-General Command had resumed hostilities in Yarmuk.
"The truce has been broken," Observatory director Rami Abdel Rahman told AFP.
The ceasefire had taken hold on February 10 when Al-Nusra withdrew its fighters from Yarmuk after months of fierce battles between rebels and forces loyal to Syria's President Bashar al-Assad.
The army laid siege to the camp, trapping tens of thousands of people inside, including Palestinian refugees and Syrians who had fled violence from other parts of the country.
The camp's population shrunk to 40,000 from more than 150,000 and conditions deteriorated to the extent that residents were forced to eat grass to survive, with some dying of starvation, activists have said.
In January the UN Relief and Works agency (UNRWA) began distributing aid in Yarmuk after clinching a deal with the warring parties, with 7,500 food parcels handed out since then.
At least one person was killed in Sunday's renewed violence, the Observatory said.
Activists said the fighting and shelling erupted when Al-Nusra jihadists returned to Yarmuk.
"I was out filming and suddenly the shelling started. You should have seen the children: they were terrified," said activist Rami al-Sayed.
UNRWA spokesman Chris Gunness deplored the fighting and called for it to end in order to allow the resumption of aid operations.
"UNRWA demands that all parties in Yarmuk cease hostilities and seek to resolve their differences exclusively by peaceful mean," he said on Tweeter.
- 'Everything now calm' -
In Moadamiyet al-Sham, southwest of the capital, families displaced by fighting were returning to inspect their homes, visit relatives and consider if it was safe to return for good.
Moadamiyet al-Sham was once home to 100,000 people, but fighting, bombing and an army blockade forced tens of thousands to seek shelter elsewhere.
In December, rebels and President Assad's regime agreed a truce, after the town had been besieged for more than a year by government forces.
"We've been told everything is calm now, so we've decided to return just to see the house," a mother of two who fled 14 months ago told AFP at an army checkpoint on the edge of the town.
Some 15,000 people still live in Moadamiyet al-Sham, where aid has trickled in since the truce was clinched, but basic services apparently are still lacking.
"Electricity is still cut off inside," said another women, accompanied by her four children.
In Madrid, El Periodico newspaper said Marginedas, who was abducted by jihadists on September 4 in central Syria, had been released at dawn and was undertaking medical tests in Turkey.
The UN Children's Fund UNICEF said on Sunday that mass vaccinations against polio had been launched in Egypt, Iraq, Jordan and Syria and an operation was to get underway March 9 in Lebanon.
Inside Syria, the campaign was targeting 1.6 million children, it said.