He added: "The Security Council resolution route has always been the best way to proceed because it does give you the best chance of getting your hands on those you want to hold to account."
However, there was "still some hope", he said, as the countries driving the tribunal would "look at the options".
But he criticised the Security Council's lack of action on the big issues facing the global community, telling the programme it "fails far too often".
"As I said it failed in relation to the Malaysian airliner - it should have been a simple matter of saying, 'we're in favour of accountability', because there's only one alternative and that's impunity for those who carried out this act.
"But big questions like Syria - five years now, 220-odd thousand people have died. The council had a discussion about chemical weapons which is being a bit more fruitful, the humanitarian discussions take place, but actually someone's got to stop the fighting.
"That's where the big guys on the council have got to get over themselves a bit. We try to encourage that without being unduly rude about it, but we've been trying to encourage them to understand that the expectations of the international community are not being met.
"You can't have this on-going saga of Syria, Iraq, Yemen, Libya, with the council declaring itself to be paralysed. We've got to find a better way of getting the council to perform."
NZME pd