From The San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994:
...Neutral merchant vessels
67. Merchant vessels flying the flag of neutral States may not be attacked unless they:
(a) are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search or capture;
(b) engage in belligerent acts on behalf of the enemy;
(c) act as auxiliaries to the enemy s armed forces;
(d) are incorporated into or assist the enemy s intelligence system;
(e) sail under convoy of enemy warships or military aircraft; or
(f) otherwise make an effective contribution to the enemy s military action, e.g., by carrying military materials, and it is not feasible for the attacking forces to first place passengers and crew in a place of safety. Unless circumstances do not permit, they are to be given a warning, so that they can re-route, off-load, or take other precautions.
...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....