Israel keeps talking about their right to blockade under international law. But that is rubbish unless something on the HUMANITARIAN AID ship violates a published list of banned items. First off, humanitarian aid is excepted by the same international law that Israel hides behind. Second, unless there is something among the humanitarian aid that is on a list of banned items that humanitarian aid is not allowed to deliver, Israel cannot stop that aid. But Israel hasn't published any such list, so Israel cannot object to anything on these aid boats, and therefore has no legal right to stop any of them.
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from the article: Israel to UN: We reserve the right to stop Lebanon Gaza-bound flotilla