The discussions on Syria at the G20 summit in St Petersburg may have gone on well past midnight on the final night, but they failed to break the tense stalemate on whether and how the international community should intervene.

The United States and Russia are still at opposite ends of the argument, with President Obama pushing hard for a strike and President Putin insisting such a strike would be counter-productive and end up destabilising the region and making a bad situation even worse.

The issue that is prolonging the stalemate is the lack of definitive evidence on whether chemical weapons have been used by President Bashar al Assad's regime. President Obama says there is no question they were and that the Syrian government was responsible for the poison gas attack in Damascus that killed almost 1500 people last month. Mr Putin, on the other hand, has suggested the attack could have been orchestrated by the rebels in an attempt to bring about the very military action that is now under consideration.

The truth is: we do not know for certain and until we do, a military strike should be out of the question. US Secretary of State John Kerry says he knows the regime used chemical weapons in Damascus and the world agrees with him. This is not so. Most governments, including the British, believe it is likely, but the main reason David Cameron lost the Commons vote on military intervention was that the intelligence is inconclusive - and who can blame MPs for being nervous about authorising the use of military force on the basis of inconclusive evidence? They remember Iraq.

The US Government's response is to insist that Syria is different from Iraq. They say the evidence is there this time and that, unlike Iraq, they have a clear plan for a targeted strike to undermine Assad's ability to use chemical weapons. That sounds fine in principle, but what is this evidence? And as for the reassurance that action in Syria would not lead to an open-ended conflict, we have heard that one before haven't we?

What is not in doubt is that the US Government is concerned about the use of chemical weapons and their proliferation and Mr Kerry is right to say that failing to act when a regime like Assad's uses such weapons opens up wider dangers for us all, although precipitate military action in the region opens up just as many dangers, not least the possible counter-reaction from Hezbollah and Iran.

And there is another danger just as great as that of acting too soon and that is the danger of not acting at all, or dithering while more Syrians die. The weapons inspectors have to be allowed to finish their job, but the search for a consensus cannot drag on forever. Yes, the evidence on the use of chemical weapons is inconclusive, but the evidence on other atrocities committed by Assad is beyond doubt. What it means is that the case for military action is strong but incomplete and we must wait until it is complete before acting. The dodgy dossier on Iraq taught us that.