Beirut - The top UN official in Syria on Monday demanded immediate and unconditional humanitarian access to tens of thousands of people trapped in four towns, warning of starvation.The International Committee of the Red Cross meanwhile released drone footage showing extensive damage to Ramadi, which was taken back from the Islamic State group in December after heavy fighting that devastated much of the Iraqi city.Aid must be allowed to reach Madaya, Zabadani, Foua and Kafraya, the UN's resident co-ordinator, Yacoub El Hillo, said in Damascus. Madaya and Zabadani, just outside the capital, are encircled by pro-government forces, while rebels are blockading Foua and Kafraya, in the country's northwest.The towns have been besieged since last year, with aid convoys allowed only sporadically to replenish food and medical stocks. The last delivery was made in April.Aid group Doctors Without Borders says 16 people died in Madaya from siege conditions in January, even after aid was allowed in. El Hillo said the UN is "calling on all parties involved to ensure this doesn't happen again."El Hillo also urged combatants to allow medical evacuations. Activists in Madaya have launched a campaign to evacuate the journalist Abdelwahab Ahmad, who was hospitalised from a bullet wound last week.Aid deliveryAhmad had drawn attention to the siege through a media campaign last December. The images and clips of emaciated children transmitted from the town sparked an international outcry.Wafiqa Hashem, a schoolteacher inside Madaya, said residents were burning blankets and clothes in their cook stoves after running out of other sources of fuel.The UN says 62 000 people are trapped in the four towns. Their fates are linked through a reciprocal agreement between rebel groups and the Syrian government. For each medical evacuation from a government-besieged town, for example, a patient must be evacuated from a town besieged by rebels, and vice versa.UN officials have said the agreement obstructs aid delivery and El Hillo said it should be scrapped.An estimated half million people are trapped in 18 areas the UN classifies as besieged, though the independent monitoring group Siege Watch puts the number at one million. Pro-government forces are responsible for most of the sieges, according to observers.