A UN report has concluded that recent US airstrikes on vessels allegedly trafficking drugs breached international law and could constitute crimes against humanity.

The report by Special Rapporteur Ben Saul said since September last year there had been at least 68 US strikes in the Caribbean and Pacific, outside US territorial waters, which killed more than 220 people.

Saul said that even if targets could be identified as members of designated terrorist groups, they had not staged an armed attack on the US which would trigger a right to self-defence under international law.

The report comes two days after the US military said it killed four people in a strike on a vessel in the Caribbean.