By Saad Sayeed, Andrew Mills, Ariba Shahid and Olivia Le PoidevinISLAMABAD, June 18 (Reuters) - Pakistani mediators spent weeks juggling late-night calls and competing drafts before a Qatari push helped secure this week's preliminary U.S.-Iran deal, but turning it into a permanent agreement will prove harder, sources said.

Mediators spent weeks working on a plan to bridge nearly half a century of hostility between Washington and Tehran

While Pakistan remained a broker, diplomats involved in the talks pointed out that Qatar increasingly became the bridge between Washington and Tehran.