Damascus - The top UN official in Damascus has warned of a "looming humanitarian catastrophe" in four besieged towns in Syria, calling for immediate access to deliver aid to some 60 000 residents.In a statement late on Monday, the UN humanitarian coordinator for Syria, Ali al-Za'atari, warned of dire conditions in the towns of Zabadani, Madaya, Fua and Kafraya.Zabadani and Madaya, in Damascus province, are besieged by government troops and their allies, while Fua and Kafraya are under siege by the rebels."Sixty thousand innocent people are trapped there in a cycle of daily violence and deprivation, where malnutrition and lack of proper medical care prevail," the statement said."The situation is a looming humanitarian catastrophe. The principle of free access to people in need must be implemented now and without repeated requests," it added.Za'atari said the situation was complicated by the "tit-for-tat arrangement" between the towns, whereby no aid can be provided to Madaya and Zabadani without similar access to Fua and Kafraya, and vice versa.The linkage "makes humanitarian access prone to painstaking negotiations that are not based on humanitarian principles," he said."This has prevented medical cases from receiving proper treatment and evacuation. People are in need, and they cannot wait any longer. We need to act now."The UN's last humanitarian access to the four towns was in November, the statement said, without directing blame for the lack of access at one side or the other.Earlier this month, the UN said it had been able to deliver aid to just 40 000 people in besieged and hard-to-reach areas in January, despite requesting access to more than 900 000 people.That made January the worst month for humanitarian deliveries in nearly a year, with approval received for just one of 21 humanitarian convoys proposed by the UN, the International Committee of the Red Cross and the Syrian Arab Red Crescent.The UN says 4.72 million Syrians are in so-called hard-to-reach areas, including 600 000 people under siege, mostly by the Syrian army, but also by rebel groups or the Islamic State group.