The highest-level talks between the two countries since 1979 ended in the Pakistani capital without a breakthrough at the weekend, raising doubts over the survival of a two-week ceasefire that still has a week to run.

But a source involved in the talks said on Tuesday both countries could return as early as the end of this week, and that a proposal had been shared ​with Washington and Tehran to resend their delegations.

"No firm date has been set, with the delegations keeping Friday through Sunday open," a senior Iranian source said. US President Donald Trump said Iran had ​been in touch on Monday and wanted to make a deal, adding that he would not sanction any agreement that allowed Tehran to possess a nuclear weapon.