Cameron's assertion that Syrian president is 'part of the problem, not part of the solution' leaves Assad out in the cold

In the fourth year of the war in Syria, Bashar al-Assad would like nothing better than to be embraced by the west as an indispensable partner in a new "war on terror" - now the focus of intense talks following advances by the Islamic State (Isis) in Iraq and the beheading of two American journalists. It doesn't look as if that is going to happen.

David Cameron's suggestion at the Nato summit that Isis targets in Syria could be attacked without cooperation with Assad clearly reveals the direction of thinking in London and Washington: in the UK prime minister's formulation, the Syrian president is "part of the problem, not part of the solution," while Assad's record of war crimes obviates any need to consult him. And both the US and Britain, after all, have long called for his removal from power.

Assad's argument since the start of the conflict in 2011 has been that he is a bulwark of stability, the only barrier to an al-Qaida takeover. Syrian state media, focusing on Gulf funding, has portrayed all his enemies as fanatical jihadis even when demonstrations were largely peaceful. At the same time, the authorities freed Islamist prisoners to bolster the official narrative and divide opposition ranks. The Syrian military refrained from attacking Isis, especially in its Raqqa stronghold in the north-east, until after the fall of the Iraqi city of Mosul in June. It also intervened to help Isis fight other rebel brigades.

Sensing a new opportunity after the killing of James Foley, Walid Muallem, Assad's veteran foreign minister, made the pitch explicitly: "Syria is ready to cooperate and coordinate ... [at] the regional and international level in the war on terror," he pledged.

On the face of it this was a tempting offer: Syria's security services are masters of the penetration and manipulation of jihadi groups which they helped cross into Iraq to fight US forces after the 2003 invasion.

And western governments have been putting out discreet feelers to Damascus for months – seeking intelligence to deal with the increasingly alarming issue of foreign fighters among the rebel ranks, as Muallem's deputy, Faisal Mekdad, confirmed to the Guardian in July. But "security matters could not be separated from the political cooperation," he insisted. The prize was always Assad's rehabilitation.

The pragmatic western case for working with the Syrian president is that the war is at a stalemate and his cooperation is vital in the face of the Isis menace.

"Sometimes you have to develop relationships with people who are extremely nasty in order to get rid of people who are even nastier," argued Sir Malcolm Rifkind, the former UK foreign secretary. Senior US figures made the same point about how "your enemy's enemy" can become your friend.

Obama and Cameron are not buying this. Additional arguments deployed against engagement with Assad are that he cannot be trusted and that helping bolster his position would alienate Sunnis in Iraq and Syria whose support is needed to fight Isis. In the words of Nadim Shehadi, the Chatham House analyst, the Syrian leader has all the credibility of a convicted arsonist offering his services as a firefighter.

Syria's fragmented opposition, understandably worried about any western attempt to engage with Assad, portrays the regime and Isis as twin threats which the international community should help the Free Syrian Army fight.

For the moment, though, the focus is relentlessly short-term. Signals from Washington suggest the US, with Nato or Arab help, would not face insurmountable difficulties mounting air strikes against Isis in Syria. Experts say talk of a "formidable" air defence system is exaggerated. The fact that US special forces were able to infiltrate Syria by air this summer in an unsuccessful mission to free hostages suggests it is less than effective. And Israel, an intimate US ally, is likely to have real-time intelligence it would gladly share with the Americans. Syria also has a record of thinking carefully before retaliating.

Assad will not welcome US-led air strikes on Isis on either side of the now irrelevant border between Syria and Iraq – unless they form part of a western rapprochement with him. Attacks will strengthen his opponents because the jihadis now control territory seized from other Syrian rebels. But he may not be able to stop them. The Damascus regime "will continue to view (Isis) as a useful tool for its survival," argues the exiled Syrian commentator Hassan Hassan. "It might be interested in degrading the group's assets but not in its elimination."