Firefights and shelling rocked the Palestinian camp of Yarmuk in southern Damascus on Sunday, breaking a truce that had held since mid-February, said a group that monitors the Syrian conflict, Alarabiya reported.
"There was shelling of Yarmuk and fighting between (the al-Qaeda-linked) al-Nusra Front and the Popular Front for the Liberation of Palestine-General Command," the Syrian Observatory for Human Rights said.
"The truce has been broken," its director Rami Abdel Rahman told AFP.
Each side blamed the other for breaking the ceasefire.
A fragile truce in army-besieged Yarmuk, where some 40,000 people have been living in inhumane conditions since last summer, had allowed in limited aid by the UN agency for Palestinian refugees in recent weeks.
The truce began after al-Nusra Front withdrew its fighters from the camp, but on Sunday it issued a statement accusing President Bashar al-Assad's regime and its allies the PFLP-GC of breaking their side of the deal.
The statement said the jihadists had agreed to withdraw because of the poor humanitarian situation in the camp, and that "a very small number of food parcels" had been distributed in comparison to the agreed amounts.
Activists in Yarmuk confirmed via the Internet that al-Nusra Front members re-entered the camp Sunday, and also reported shelling and fighting there.
"I was out filming and suddenly the shelling started. You should have seen the children, they were terrified," said Rami al-Sayed, a Syrian activist living in Yarmuk, who was reached via the internet.
The pro-Assad PFLP-GC blamed Al-Nusra Front for breaking the accord by re-entering the camp.
"This morning, al-Nusra Front returned once again to Yarmuk camp, disrupting the peace initiative whose aim was to address the tragedy of the hostage camp," said spokesman Anwar Raja.
UN Relief and Works Agency (UNRWA) spokesman Chris Gunness meanwhile said via Twitter: "Bad news: looks like there were no UNRWA food distributions today in the besieged Palestinian refugee camp of Yarmuk."
Since January, UNRWA has distributed little more than 7,500 food parcels in the camp, calling this "a drop in the ocean compared with the rising tide of need."
After months of shelling and fierce fighting in and around Yarmuk between rebels and Assad loyalists, the camp's population has shrunk from more than 150,000 to 40,000. Among them are 18,000 Palestinians.