“The continued obstruction and withholding of humanitarian aid in Syria is illegal," Bayley stated.

“The fact is the vast majority of areas are besieged by the Syrian regime and in many cases the regime has refused to allow any aid in at all. There is only one explanation for this: Asad [sic] is using starvation and suffering as a weapon of war,” he added.

Special Representative Bayley said that all members of the International Syria Support group (ISSG) -- which includes the U.S., Russia and the EU -- had committed to use their influence with parties on the ground to ensure access for humanitarian assistance across Syria, as a UN resolution.

“Despite this agreement the Asad [sic] regime has continued to prevent the delivery of aid convoys, remove medical supplies, prevent access of medical personnel, and refuse medical evacuations,” Bayley said.

Bayley said that airdrops to deliver aid to all designated besieged areas remained a last resort as "an expensive and complex way to deliver aid”.

He noted that ISSG members had committed on May 17 to make a final effort to persuade the regime to let aid in, by land, by Wednesday and called on Russia and Iran to make use of their influence.

Syria has been locked in a vicious civil war since early 2011, when the regime cracked down on pro-democracy protests.

Since then, more than 250,000 people have been killed, according to UN figures.

Anadolu Agency