Some 30 celebrities have petitioned the UN demanding they take action to protect civilians, notably Palestinian refugees, caught up in the Syria conflict as it entered its fourth year on Saturday.
Stars including Hugh Grant, Emma Thompson, Sting, Roger Waters and Tracey Emin were among those calling on the United Nations Security Council and presidents of the General Assembly to create safe spaces and local ceasefires to allow emergency supplies to reach besieged people.
The letter -- signed chiefly by British figures from the world of film, literature, acting, music and visual arts -- is endorsed by the UN agency for Palestinian refugees UNRWA, and the HOPING charity which supports Palestinian refugee children.
Nearly 500,000 Palestinians were registered as refugees in Syria before the civil war. UNRWA has said the majority of them are displaced from their normal homes.
Most of the 170,000 residents in the Yarmuk Palestinian refugee camp on the outskirts of Damascus residents have fled, but nearly 40,000 remain trapped inside, living in abject conditions.
The celebrities asked the UN bodies "to tell us how they will create safe spaces and local ceasefires with immediate effect.
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"People and relief must be let through. We want our voices to carry, on behalf of those now suffering in Syria, in order to prevent any more tragedy."
Among the celebrity signatories were musicians Bryan Adams, Peter Gabriel and Annie Lennox; artists Tracey Emin, Carsten Hoeller and Jeremy Deller.
They also included film directors Alfonso Cuaron, Ken Loach and Stephen Frears; and writer Hanif Kureishi.
UNRWA spokesman Chris Gunness said: "It is now exactly two weeks since our last delivery of humanitarian aid to Yarmouk refugee camp in Damascus. The situation there is desperate, so this appeal is timely and urgent.
"The people of Yarmouk and civilians across Syria urgently need these UN bodies to heed the call of these celebrities.
"We are getting continued reports of widespread malnutrition, particularly among children, of women dying in childbirth for lack of medical care and people reduced to eating animal feed."
Syria's civil war enters a fourth year on Saturday, with at least 146,000 people dead, millions more homeless, cities and historical treasures in ruins, the economy devastated and no end in sight.