Palestinian refugees are being starved, bombed and gunned down like animals. “If you want to feed your children, you need to take your funeral shroud with you,” one told Israeli news website Ynet. “There are snipers on every street, you are not safe anywhere.” This isn’t happening, however, in southern Lebanon, or even Gaza. And these particular Palestinians aren’t being killed or maimed by Israeli bombs and bullets. This is Yarmouk, a refugee camp on the edge of Damascus, just a few miles from the palace of Bashar al-Assad. Since 1 April, the camp has been overrun by Islamic State militants, who have begun a reign of terror: detentions, shootings, beheadings and the rest. Hundreds of refugees are believed to have been killed in what Ban Ki-moon has called the “deepest circle of hell”.

But this isn’t just about the depravity of Isis. The Palestinians of Yarmouk have been bombarded and besieged by Assad’s security forces since 2012. Water and electricity were cut off long ago, and of the 160,000 Palestinian refugees who once lived in the camp only 18,000 now remain. The Syrian regime has, according to Amnesty International, been “committing war crimes by using starvation of civilians as a weapon”, forcing residents to “resort to eating cats and dogs”. Even as the throat-slitters took control, Assad’s pilots were continuing to drop barrel bombs on the refugees. “The sky of Yarmouk has barrel bombs instead of stars,” said Abdallah al-Khateeb, a political activist living inside the camp.

It is difficult to disagree with the verdict of the Palestinian League for Human Rights that the Palestinians of Syria are “the most untold story in the Syrian conflict”. There are 12 official Palestinian refugee camps in Syria, housing more than half a million people. Ninety per cent, estimates the United Nations Relief and Works Agency (Unrwa), are in continuous need of humanitarian aid. In Yarmouk, throughout 2014, residents were forced to live on around 400 calories of food aid a day – fewer than a fifth of the UN’s recommended daily amount of 2,100 calories for civilians in war zones – because UNRWA aid workers had only limited access to the camp. Today, they have zero access.“To know what it is like in Yarmouk,” one of the camp’s residents is quoted as saying on the UNRWA website, “turn off your electricity, water, heating, eat once a day, live in the dark.”

Their plight should matter to us all – regardless of whether their persecutors happen to be Israelis, Syrians, Egyptians or, for that matter, fellow Palestinians (Palestinian Authority security forces, after all, have been shooting and beating unarmed Palestinian protesters for several years now).

This is far from a cynical exercise in pro-Israeli whataboutery. There are very good reasons that Israel attracts such widespread criticism and condemnation in the west. Israel is our ally and claims to be a liberal democracy, unlike both Assad and Isis. Israel is also armed, funded and protected from UN censure by the US government; again, unlike both Assad and Isis.

Those who try to use the tragedy of Yarmouk to excuse or downplay Israel’s 48-year occupation of Palestine should be ashamed of themselves. But what of the rest of us? Can we afford to stay in our deep slumber, occasionally awakening to lavishly condemn only Israel? Let’s be honest: how different, how vocal and passionate, would our reaction be if the people besieging Yarmouk were wearing the uniforms of the IDF?

Our selective outrage is morally unsustainable. Many of us who have raised our voices in support of the Palestinian cause have inexcusably turned a blind eye to the fact that tens of thousands of Palestinians have been killed by fellow Arabs in recent decades: by the Jordanian military in the Black September conflicts of the early 1970s; by Lebanese militias in the civil war of the mid-1980s; by Kuwaiti vigilantes after the first Gulf war, in the early 1990s. Egypt, the so-called “heart of the Arab world”, has colluded with Israel in the latter’s eight-year blockade of Gaza.

Meanwhile, the Palestinians of Yarmouk are living in catastrophic conditions, their lives “profoundly threatened”, in the words of the United Nations. So what, if anything, can be done? The usual coalition of neoconservative hawks and so-called liberal interventionists in the west want to bomb first and ask questions later, while the rest of us resort to a collective shrug: a mixture of indifference and despair. Few are willing to make the tough and unpopular case for a negotiated solution to the Syrian conflict or, at least, a truce and a ceasefire, a temporary cessation of hostilities. Yet there is an urgent need for a “pause” in the fighting in order to ensure “humanitarian access” to Yarmouk, says Chris Gunness, senior director of UNRWA, who has described the camp as a hellhole.

UNRWA, he tells me, is “calling on those who can influence the parties on the ground to make that influence effective”, adding: “Everyone in the Middle East can be influenced, everyone is sponsored.” Gunness points out that almost 100 civilians, including 20 children, were evacuated from the camp on 5 April so there is no reason why more of Yarmouk’s residents can’t be escorted to safety.

We have also failed to put our money where our collective mouth is. The UN’s $415m appeal for Palestinian refugees in Syria is only 20% funded, a situation Gunness calls “disastrous”. Isn’t it a scandal that there’s always spare cash for bombing campaigns yet never enough for emergency aid? The Palestinians of Yarmouk, like the Palestinians of Gaza during the summer of 2014, need our support, both political and financial.

Now is the time for those of us who claim to care about the Palestinian people, and their struggle for dignity, justice and nationhood, to make our voices heard. Some 3,500 of the 18,000 Palestinians in Yarmouk are children. As Gunness says, his voice trembling with emotion: “We are potentially witnessing a slaughter of the innocents. What is the world going to do?”

• The headline of this article was amended on Monday 13 April 2015