After Iran talks falter, the big question is 'what happens next?'

Twenty-one hours was not enough to end 47 years of hostility between Iran and the US, writes the BBC's Lyse Doucet.

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The historic high-level talks in Islamabad, during a pause in weeks of grievous war, were always unlikely to end any other way.

Calling this marathon negotiating session a failure belies the scale of the challenge in narrowing wide gaps on complex issues ranging from age-old suspicion about Iran's nuclear programme to new challenges this war has thrown up - most of all Iran's control of the strategic Strait of Hormuz, whose closure is causing economic shocks worldwide.

To do a deal, they also needed to overcome a deep chasm of distrust.

A day ago, it wasn't even certain the two sides would meet, and even more, sit down in the same room.