ISLAMABAD/WASHINGTON/DUBAI, April 15 -- Negotiating teams from the U.S. and Iran could return to Islamabad this week to resume talks to end the war, sources told Reuters on Tuesday, after the collapse of weekend negotiations prompted Washington to impose a blockade on Iranian ports. While the US blockade drew angry rhetoric from Tehran, signs that diplomatic engagement might continue helped calm oil markets, pushing benchmark prices below $100 on Tuesday. The highest-level talks between the two adversaries since the 1979 Islamic Revolution 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. "No firm date has been set, with the delegations keeping Friday through Sunday open," a senior Iranian source said....