Turkey has directly supported al-Qaeda's wing in Syria, in defiance of America, the former US ambassador has disclosed.

The Turkish authorities thought they could work with extremist Islamist groups in the Syrian civil war and at the same time push them to become more moderate, Francis Ricciardone, who was until late June the US ambassador to Ankara, told journalists in a briefing. That led them to work with Jabhat al-Nusra, an al-Qaeda offshoot, as well as hardline Salafi Islamist groups such as Ahrar al-Sham. Mr Ricciardone said that he tried to persuade the Turks to close their borders to the groups, but to no avail.

Islamic State militants.

"We ultimately had no choice but to agree to disagree," he said. "The Turks frankly worked with groups for a period, including al-Nusra, whom we finally designated as [being groups] we're not willing to work with." Turkey allowed its borders to be used as a conduit for aid, weapons and volunteers heading to support Syrian rebels from the start of the uprising, and there have long been accusations that it did not do enough to distinguish between "moderate" groups and extremists.

But this is the first time a senior American official - albeit one no longer in service - has said openly that Turkey was working with al-Qaeda. Ironically, the Turkish policy has been effective in one way - Jabhat al-Nusra is now seen as relatively moderate compared with its splinter group, Islamic State. But in other respects, it has backfired.