30 MEDVEDYEV AND OTHERS v. FRANCE JUDGMENT customary law of the sea, its provisions concerning illicit traffic in narcotic drugs on the high seas – like those of the complementary Vienna Convention, organising international cooperation without making it mandatory – reflect a lack of consensus and of clear, agreed rules and practices in the matter at the international level. 93. The Court notes, however, that independently of the Montego Bay and Vienna Conventions, and of French law, Cambodia consented in a diplomatic note to the intervention of the French authorities, a fact which, according to the Government, attested to the existence of an ad hoc agreement between the two countries on the interception of the Winner and the subsequent events. 94. The question is therefore whether the diplomatic note of the Ministry of Foreign Affairs of Cambodia dated 7 June 2002 provided a legal basis for the impugned measures. 95. In the Court’s opinion, although the provisions of Article 108 § 2 of the Montego Bay Convention do not apply to the present case, as Cambodia has not ratified that instrument, they do not prevent States from envisaging other forms of collaboration to combat drug trafficking at sea. The United Nations Single Convention on Narcotic Drugs, 1961 (see paragraph 27 above, Article 35 (c)) and the Montego Bay and Vienna Conventions (see paragraphs 28-29 above, Article 108 § 1 and Article 17 § 1 respectively) all provide expressly for cooperation between States on this matter. Such cooperation may take various forms, particularly in view of the vague wording of the provisions of Article 17 § 4 (c), which merely refers to “appropriate action”, and give rise, for example, to regional agreements, like the Council of Europe Agreement of 31 January 1995 implementing Article 17 of the Vienna Convention (see paragraph 30 above) and the San José Agreement of 10 April 2003 on regional cooperation in the Caribbean (see paragraphs 31-33 above), or to the bilateral treaties referred to in Article 17 § 9 of the Vienna Convention. 96. Moreover, diplomatic notes are a source of international law comparable to a treaty or an agreement when they formalise an agreement between the authorities concerned, a common stance on a given matter or even, for example, the expression of a unilateral wish or commitment. 97. The Court accordingly considers, like the Government, that the diplomatic note issued by the Cambodian authorities on 7 June 2002 officialised Cambodia’s agreement to the interception of the Winner, Cambodia having the right to engage in cooperation with other countries outside the framework of the Montego Bay and Vienna Conventions. 98. However, the existence of an ad hoc agreement does not solve the problem of its scope, which it is for the Court to appreciate in order to determine whether or not the diplomatic note authorised the arrest and detention of the crew members on board the ship and their transfer to France.