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San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994
METHODS OF WARFARE
SECTION II : METHODS OF WARFARE
93. A blockade shall be declared and notified to all belligerents and neutral States.
94. The declaration shall specify the commencement, duration, location, and extent of the blockade and the period within which vessels of neutral States may leave the blockaded coastline.
95. A blockade must be effective. The question whether a blockade is effective is a question of fact.
96. The force maintaining the blockade may be stationed at a distance determined by military requirements.
97. A blockade may be enforced and maintained by a combination of legitimate methods and means of warfare provided this combination does not result in acts inconsistent with the rules set out in this document.
98 Merchant vessels believed on reasonable grounds to be breaching a blockade may be captured. Merchant vessels which, after prior warning, clearly resist capture may be attacked.
99. A blockade must not bar access to the ports and coasts of neutral States.
100. A blockade must be applied impartially to the vessels of all States.
101 The cessation, temporary lifting, re-establishment, extension or other alteration of a blockade must be declared and notified as in paragraphs 93 and 94.
102 The declaration or establishment of a blockade is prohibited if:
(a) it has the sole purpose of starving the civilian population or denying it other objects essential for its survival;
(b) the damage to the civilian population is, or may be expected to be, excessive in relation to the concrete and direct military advantage anticipated from the blockade.
103. If the civilian population of the blockaded territory is inadequately provided with food and other objects essential for its survival, the blockading party must provide for free passage of such foodstuffs and other essential supplies, subject to:
(a) the right to prescribe the technical arrangements, including search, under which such passage is permitted;
(b) the condition that the distribution of such supplies shall be made under the local supervision of a Protecting Power or a humanitarian organization which offers guarantees of impartiality, such as the International Committee of the Red Cross.
104. The blockading belligerent shall allow the passage of medical supplies for the civilian population or for the wounded and sick members of armed forces, subject to the right to prescribe technical arrangements, including search, under which such passage is permitted.
105. A belligerent cannot be absolved of its duties under international humanitarian law by establishing zones which might adversely affect the legitimate uses of defined areas of the sea.
106. Should a belligerent, as an exceptional measure, establish such a zone:
(a) the same body of law applies both inside and outside the zone;
(b) the extent, location and duration of the zone and the measures imposed shall not exceed what is strictly required by military necessity and the principles of proportionality;
(c) due regard shall be given to the rights of neutral States to legitimate uses of the seas;
(d) necessary safe passage through the zone for neutral vessels and aircraft shall be provided:
(i) where the geographical extent of the zone significantly impedes free and safe access to the ports and
coasts of a neutral State;
(ii) in other cases where normal navigation routes are affected, except where military requirements do
(e) the commencement, duration, location and extent of the zone, as well as the restrictions imposed, shall be publicly declared and appropriately notified.
107. Compliance with the measures taken by one belligerent in the zone shall not be construed as an act harmful to the opposing belligerent.
108. Nothing in this Section should be deemed to derogate from the customary belligerent right to control neutral vessels and aircraft in the immediate vicinity of naval operations.