The U.S. military began a blockade of Iran's ports on Monday, President Donald Trump said, and Tehran threatened to ​retaliate against its Gulf neighbors' ports after weekend talks in Islamabad on ending the war broke down, Reuters reported. 

A U.S. official said there was continued engagement with Iran, and forward motion on ‌trying to get to an agreement. Pakistani Prime Minister Shehbaz Sharif also said efforts were still under way to resolve the conflict.

But oil prices climbed back to $100 per barrel, with no sign of a swift reopening of the Strait of Hormuz to ease the biggest ever disruption in supplies and broader concerns over the durability of a two-week ceasefire agreement reached last week.