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Within the limits of their regional competence and to the extent of their powers and ability, the customs authorities of one State shall, at their own initiative or at the request of the customs authorities of the other State, maintain surveillance over:
(a) the movements, particularly the entry into and exit from their territory, of persons who have or are suspected of having committed offences against customs laws of the other State or who are suspected of committing such offences;
(b) vehicles, ships, aircraft and other means of transport which have been or are suspected of having been used for committing offences against customs laws of the other State or which are suspected of being used to commit such offences;
(c) movements of goods which are reported by the customs authorities of the other State as giving rise to substantial illicit traffic to their territory;
(d) places where unusual stocks of goods have been built up, giving reason to assume that they are to be used for illicit importation into the territory of the other State.