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San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994
OTHER ENEMY VESSELS AND AIRCRAFT
SECTION IV : OTHER ENEMY VESSELS AND AIRCRAFT
Enemy merchant vessels
59. Enemy merchant vessels may only be attacked if they meet the definition of a military objective in paragraph 40.
60. The following activities may render enemy merchant vessels military objectives:
(a) engaging in belligerent acts on behalf of the enemy, e.g., laying mines, minesweeping, cutting undersea cables and pipelines, engaging in visit and search of neutral merchant vessels or attacking other merchant vessels;
(b) acting as an auxiliary to an enemy s armed forces, e.g., carrying troops or replenishing warships;
(c) being incorporated into or assisting the enemy s intelligence gathering system, e.g., engaging in reconnaissance, early warning, surveillance, or command, control and communications missions;
(d) sailing under convoy of enemy warships or military aircraft;
(e) refusing an order to stop or actively resisting visit, search or capture;
(f) being armed to an extent that they could inflict damage to a warship; this excludes light individual weapons for the defence of personnel, e.g., against pirates, and purely deflective systems such as chaff ;
(g) otherwise making an effective contribution to military action, e.g., carrying military materials.
61. Any attacks on these vessels is subject to the basic rules set out in paragraphs 38-46.
Enemy civil aircraft
62. Enemy civil aircraft may only be attacked if they meet the definition of a military objective in paragraph 40.
63. The following activities may render enemy civil aircraft military objectives:
(a) engaging in acts of war on behalf of the enemy, e.g., laying mines, minesweeping, laying or monitoring acoustic sensors, engaging in electronic warfare, intercepting or attacking other civil aircraft, or providing targeting information to enemy forces;
(b) acting as an auxiliary aircraft to an enemy's armed forces, e.g., transporting troops or military cargo, or refuelling military aircraft;
(c) being incorporated into or assisting the enemy's intelligence-gathering system, e.g., engaging in reconnaissance, early warning, surveillance, or command, control and communications missions;
(d) flying under the protection of accompanying enemy warships or military aircraft;
(e) refusing an order to identify itself, divert from its track, or proceed for visit and search to a belligerent airfield that is safe for the type of aircraft involved and reasonably accessible, or operating fire control equipment that could reasonably be construed to be part of an aircraft weapon system, or on being intercepted clearly manoeuvring to attack the intercepting belligerent military aircraft;
(f) being armed with air-to-air or air-to-surface weapons;
(g) otherwise making an effective contribution to military action.
64. Any attack on these aircraft is subject to the basic rules set out in paragraphs 38-46.
Enemy warships and military aircraft
65. Unless they are exempt from attack under paragraphs 47 or 53, enemy warships and military aircraft and enemy auxiliary vessels and aircraft are military objectives within the meaning of paragraph 40.
66. They may be attacked, subject to the basic rules in paragraphs 38-46.