Iranian armed forces launched attacks on U.S. military infrastructure in neighbouring Gulf states on Thursday following U.S. strikes on Iran's southern coastal and eastern provinces, putting further strain on a three-week-old ceasefire agreement. Iran was also preparing on Thursday to bury its slain Supreme Leader Ayatollah Ali Khamenei at the country's holiest shrine in Mashhad, in the northeast, the culmination of a week of mass funeral processions and rallies. Khamenei was killed in a U.S. airstrike on the first day of the war on February 28. Iran's air force is flying MiG-29 jets to "secure the skies over the funeral procession" in Mashhad, Fars news agency said. Oil prices, which had spiked amid concerns over the impact of the renewed attacks on global supplies, fell back on Thursday as investors weighed whether the flare-up was tactical and temporary or might augur a complete collapse in the ceasefire. The U.S. military said on Wednesday its latest strikes on Iran were aimed at keeping the Strait of Hormuz open after it said Iranian forces had struck three tankers in the area. The