ISIS militants besieging a Palestinian refugee camp in Syria's capital have savagely beheaded the leaders of a Hamas-linked group opposed to President Bashar Assad, it has been claimed.

Having seized control of up to 90 per cent of the Yarmouk camp in south Damascus, the jihadis posted gruesome images showing the severed heads of two alleged members of Aknaf Beit al-Maqdis online.

Like ISIS, the Palestinian group has declared Israel its sworn enemy, although its presence in Yarmouk has until now been to defend the camp's 18,000 residents from a crippling two year siege carried out by the Assad regime.

ISIS' decision to publicly execute the men - as well as reports of the deaths of dozens more Palestinians during clashes inside Yarmouk - has prompted outrage in Gaza, where hundreds of furious Hamas supporters swore bloody revenge on the terror group during a march yesterday.

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Savages: ISIS militants (pictured) stormed the Yarmouk camp in south Damascus last Wednesday. 100 civilians were either killed or kidnapped by the jihadis in the subsequent days

Killers: Islamic State militants are seen storming the Yarmouk camp in Damascus last week

Hundreds of residents of the Yarmouk camp fled yesterday amid shelling by government forces and clashes between ISIS fighters and Palestinian militants, activists said.

An activist based in an area just south of Damascus, Hatem al-Dimashqi, said many residents started fleeing the camp after midnight as the fighting let up. The camp has been subjected to intense shelling and airstrikes by government warplanes allegedly carrying barrel bombs.

This morning activists said shelling and sporadic clashes are striking the Palestinian refugee camp in the Syrian capital, a situation a U.N. official describes as 'beyond inhumane.'

Hatem al-Dimashqi, an activist based in an area just south of Damascus, said the Yarmouk camp witnessed sporadic clashes and shelling earlier today.

The UK-based Syrian Observatory for Human Rights and al-Dimashqi said the refugees who have fled the camp have reached the southern Damascus suburbs of Yalda, Babila and Beit Sahem, which are under the control of anti-Assad rebels.

Al-Dimashqi and Syrian state TV said as many as 2,000 people have left the camp. The activist said that many of those who fled the camp are staying in schools or abandoned homes.

Roaming the streets: ISIS stormed the camp last Wednesday, marking the terrorist's deepest foray yet into Damascus - the Syrian capital and base of President Assad's government

Power: Palestinian officials and Syrian activists said ISIS have been working with rivals from the Al Qaeda affiliate in Syria, Nusra Front and have taken control of up to 90 per cent of Yarmouk

Chris Gunness, a spokesman for the U.N. agency that supports Palestinian refugees known as UNRWA, last night said that the agency has not been able to send any food nor any convoys into the camp since the current round of fighting started.

'That means that there is no food, there is no water and there is very little medicine,' he added.

ISIS stormed the camp on Wednesday, marking the terrorist's deepest foray yet into Damascus - the Syrian capital and base of President Assad's government.

Palestinian officials and Syrian activists said ISIS have been working with rivals from the Al Qaeda affiliate in Syria, Nusra Front.

The two groups have fought bloody battles against each other in other parts of Syria, but appear to be cooperating in the attack on Yarmouk.

The Nusra Front said in a statement yesterday that it is not participating in the battles and is taking a neutral stance. The statement added that Nusra opened its offices and welcomed all those who don't want to take part in the fighting and gave them refuge.

Conflict zone: Hatem al-Dimashqi, an activist based in an area just south of Damascus, said the Yarmouk camp (pictured) witnessed sporadic clashes and shelling earlier today

The UN says around 18,000 civilians, including a large number of children, are trapped in Yarmouk (pictured)

Speaking of Hamas-linked groups' clashes with ISIS in the camp, Palestinian President Mahmoud Abbas said that residents of Yarmouk have been victimized by Syria's civil war.

Government forces and different rebel factions are clashing and 'we pay the price,' he said.

He added that the Palestine Liberation Organization in Damascus has formed a 'cell to handle this tragedy and they are trying to work it out with the least losses.' Abbas said: 'We are in touch with our brothers there to find a way out and protect our people.'

The Observatory said the fighting has killed 26 people since the clashes first broke out.

In Gaza, several hundred supporters of the ruling Hamas group staged a march in the Jebaliya refugee camp late last night to protest ISIS' takeover of parts of Yarmouk.

'Palestinian blood is not cheap,' Mohammed Abu Askar, a local Hamas leader, told the crowd, threatening revenge for the Islamic State incursion into Yarmouk.

In Damascus, Anwar Raja, the spokesman for Damsascus-based Popular Front for the Liberation of Palestine - General Command, which supports embattled Syrian President Bashar Assad, said several pro-Assad factions have united to defend the camp.

He said more than 100 civilians have been either killed or kidnapped by the ISIS fighters whom he said now control about half the camp, adding that the priority now is to evacuate civilians.

Anger: Palestinians shout slogans as they hold banners during a gathering in solidarity with the Palestinians living and dying in Syria's Yarmouk camp over the weekend

Taking to the streets: In Gaza, several hundred supporters of the ruling Hamas group staged a march in the Jebaliya refugee camp late last night to protest ISIS' takeover of parts of Yarmouk

Placards: Speaking of Hamas-linked groups' clashes with ISIS in the camp, Palestinian President Mahmoud Abbas said that residents of Yarmouk have been victimized by Syria's civil war

The UN says around 18,000 civilians, including a large number of children, are trapped in Yarmouk.

The camp has been under government siege for nearly two years, leading to starvation and illnesses caused by lack of medical aid.

The camp has also witnessed several rounds of ferocious and deadly fighting between government forces and militants.

Most of the camp's estimated 160,000 inhabitants fled in late 2012 as clashes erupted between pro- and anti-Assad Palestinian gunmen.

The majority of those who left fled to overcrowded and destitute Palestinian refugee camps in neighboring Lebanon. Only the poorest remained behind.

Yesterday Lebanon's state-run National News Agency said six Lebanese truck drivers who were held by militants after they captured the Nasib border crossing point with Jordan have returned safely to Lebanon.