IS assault blocks food aid to Syria's Deir az-Zour
The militants have besieged 100,000 residents in a section of the eastern city since 2015, having already seized large parts of Deir az-Zour. On Saturday, IS militants thrust further into areas still held by the Syrian regime.
"We have put on hold the air drop operation in Deir az-Zour for security operational reasons," said WFP spokeswoman Bettina Luescher.
"There is heavy fighting ongoing in and around the landing zone... It is simply too dangerous to do this now," she told reporters in Geneva.
Over a hundred people have died in the clashes which began on Saturday, a monitor reported.
The last WFP delivery of aid to the besieged city was on Sunday, Luescher said, adding that 3,300 tonnes of food and other aid items had been delivered since the IS assault started four days ago.
The assault has split the eastern areas of government control from the west, while also cutting off the supply route from the city's military airport. This has limited the Syrian regime's ability to resupply and bring in reinforcements.
According to the UK-based Syrian Observatory for Human Rights, ten civilians were killed Tuesday by Syrian and Russian air strikes on IS-held al-Ummal district. This brings the number of civilians killed during the four-day IS assault to 37.
The SOHR describes the assault as the "most violent" attack on the city in over a year, involving waves of suicide attacks, among other tactics employed by IS.
Despite food drops by the WFP and Syria’s government since the beginning of the siege, residents of Deir az-Zour have still faced crippling shortages and rising prices.
The city, which is located in the oil-rich eastern province of the same name, has been retained by the IS group despite them having lost of large swathes of territory in northern Syria.
These losses have mainly been to Syrian Kurdish militias and Turkish-backed rebel forces fighting the IS group.