WASHINGTON/DUBAI, April 14 -- The US military began a blockade of ships leaving Iran's ports on Monday, President Donald Trump said, and Tehran threatened to retaliate against ports of its Gulf neighbours after weekend talks on ending the war broke down. Oil prices climbed back over $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. Trump said that Iran had been in touch on Monday and wanted to make a deal but that he will not sanction any agreement that allows Tehran to have a nuclear weapon."Iran will not have a nuclear weapon," Trump told reporters at the White House. "We can't let a country blackmail or extort the world." A US official said there was continued engagement between the US and Iran and cited forward motion on trying to get to an agreement. Meanwhile, Pakistan defence minister Khawaja Asif said the next round of Iran-US negotiations was expected soon, a day after the Islamabad talks failed to clinch a deal. Since the war started on February 28, Iran has effectively shut the Strait of Hormuz to all vessels except its own, saying passage would be permitted only under Iranian control and subject to a fee. Trump said earlier that Washington would block Iranian vessels and any ships that paid such tolls and that any Iranian "fast-attack" ships that went near the blockade would be eliminated. Brigadier General Reza Talaei-Nik, a spokesperson for Iran's Ministry of Defence, warned that efforts by foreign military to police the strait would escalate the crisis and instability in global energy security. NATO allies including Britain and France said they would not be drawn into the conflict by taking part in the blockade, stressing instead the need to reopen the waterway, through which about one-fifth of the world's oil normally passes. The ceasefire that halted six weeks of U.S. and Israeli airstrikes looked in jeopardy, with only a week left to run. Washington said Tehran rejected its demands at weekend talks in Islamabad, the highest-level discussions since Iran's 1979 Islamic Revolution....