DUBAI/tehran, April 23 -- Iran fired on three ships in the Strait of Hormuz on Wednesday, intensifying its assault on shipping in the waterway crucial to global energy supplies and complicating already faltering efforts to bring the United States and Iran together for talks to end the war. The attacks were carried out by the paramilitary Revolutionary Guard, according to Iranian media, which reported that the force seized two of the ships and was bringing them to Iran. That amounted to an escalation by Iran's leaders, who appear poised to drive a harder bargain with American negotiators after President Donald Trump said the US would indefinitely extend the ceasefire with Iran that had been due to expire Wednesday. Despite the extension, Trump also seemed to dig in, saying the US would continue to blockade Iranian ports. Iran opened fire on a container ship in the strait on Wednesday morning, and a second was attacked a short time later, according to the British military's United Kingdom Maritime Trade Operations Centre. "The Islamic Revolutionary Guard Corps naval force this morning identified and stopped in the Strait of Hormuz two violating ships," the Guards said in a statement. "The two offending ships... were seized by the IRGC's naval forces and directed to the Iranian coast." They identified one ship as the Panama-flagged container ship MSC Francesca and the Liberia-flagged Epaminodas. The tracking site Marine Traffic showed the last known positions of both vessels closer to the Iranian coast of the strait, northeast of Oman. The semi-official Nour News, Fars and Mehr news agencies then reported the Guard attacked a third vessel, which it said had become "stranded" on the Iranian coast, without elaborating. British Security firm Vanguard Tech identified it as the Panama-flagged containership Euphoria, which it said was "transiting outbound of the Strait of Hormuz". There have been more than 30 attacks on ships in the Mideast since the war began Feb. 28 with US and Israeli airstrikes on Iran. Iran's ability to restrict traffic through the strait - which leads from the Persian Gulf to the open ocean - has proved a major strategic advantage. While the ceasefire means that American and Israeli airstrikes have stopped in Iran - and Tehran's missiles no longer target Israel and the wider Middle East - the attacks in the strait and earlier American interdictions of Iranian ships show the maritime threat remains. Without any diplomatic agreement, those attacks will likely deter ships from even attempting to pass through the waterway, and further squeeze global energy supplies. Wednesday's attacks saw Brent crude oil, the international standard, spike to nearly $100 a barrel, up more than 35% since the war started. As the assaults unfolded, Iran's Revolutionary Guard vowed to "deliver crushing blows beyond the enemy's imagination to its remaining assets in the region." The night before, hard-line supporters of Iran's theocracy held rallies in which the Guard showed off missiles and launchers - a sign of defiance to Israel and the U.S., which devoted much of their airstrike campaign to destroying the county's ballistic missile arsenal....