Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 53. Starvation as a Method of Warfare
Section B. Sieges that cause starvation
The UK LOAC Manual (2004) states:
5.34. The principles of the law of armed conflict, particularly the rules relating to attacks, apply equally to situations of siege or encirclement.
5.34.2. Encirclement poses problems for the attacking commander if there are civilians in the encircled area, especially if the encircled area is, or contains, a town which is inhabited by civilians but is defended by enemy armed forces. In any bombardment, the normal rules on precautions in attack apply. So do the rules preventing starvation of the civilian population as a method of warfare or those protecting supplies indispensable for the survival of the civilian population. There is also an obligation to allow essential relief supplies through to the civilian population. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 5.34. and 5.34.2.
With regard to internal armed conflict, the manual states:
15.19. Starvation of civilians as a method of warfare is prohibited.
15.19.1. The right to life is a non-derogable human right. Violence to the life and person of civilians is prohibited, whatever method is adopted to achieve it. It follows that the destruction of crops, foodstuffs and water sources, to such an extent that starvation is likely to follow, is also prohibited. The same applies to sieges, blockades, embargoes, or the blocking of relief supplies with the intention of causing starvation. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.19–15.19.1.