By Jon Silverman
Legal affairs analyst
In international law, there seem to be two key issues arising from the capture of the 15 Royal Navy personnel by Iran.
A letter from one captive has been shown on TV
Was their vessel in Iranian territorial waters? And do they qualify for the protections afforded to prisoners of war by the Geneva Conventions? Neither question has a simple answer.
Britain believes it has made a compelling case that the patrol was within Iraqi waters when it was seized.
If that is right, then, as the British foreign secretary has pointed out, there can be no legal substance to Iranian charges of espionage because the marines were operating outside of Iran's territory, with the agreement of the government of Iraq.
Question over prisoners
Under the UN Convention on the Law of the Sea, a state has the right to set a limit of 12 nautical miles from its coast within which it exercises legal authority.
Any vessel straying within that zone is to be regarded as "innocent" as long as it is not engaged in activity "prejudicial to the peace, good order or security of the coastal state".
An armed ship or one collecting information - as the Iranians allege - might be considered to have breached the convention.
Then there is the question of the treatment of the 15 prisoners.
The crew has been held captive for more than a week
If a state of armed conflict existed between Britain and Iran, the treatment of the captured marines would fall under the provisions of the 1949 Geneva Conventions and the additional Protocols of 1977.
Article 13 of the Conventions says that POWs must be "protected against insults and public curiosity."
The framers of the Conventions were responding to the excesses of the Second World War and could not have envisaged the use of television as a means of parading captured military personnel, but most international lawyers would regard that practice as a blatant breach of humanitarian law.
However, as psychological pressure, it would not be regarded as a form of torture.
Marines' 'illegal act'
Britain, of course, is not at war with Iran so the 15 captives cannot be considered as POWs. Are they, then, hostages?
Iran would say not, because they claim the marines were engaged in an illegal act when captured.
But if they are being used as some kind of bargaining chip or to put pressure on Britain (or perhaps the United States) then their status would change and hostage-taking is a clear breach of the Geneva Conventions.
As a matter of law, this is a dispute which can probably only be settled by the International Court of Justice, which arbitrates between states.
But that would take years, so if there is to be a swift and non-violent resolution, it will be down to diplomacy and power politics to achieve it.