Srinagar, June 27 -- Srinagar - The United States launched air strikes on Iranian military targets on Friday in retaliation for what it said was an Iranian drone attack on a commercial cargo ship in the Strait of Hormuz, prompting Iran to fire back at US positions in the region and pushing a fragile ceasefire agreement between the two countries to its most serious crisis yet.

The exchange marked the biggest challenge so far to the interim memorandum of understanding (MoU) signed a week ago, under which Washington and Tehran agreed to work toward a permanent settlement to months of conflict while reopening the strategically vital Strait of Hormuz.

US President Donald Trump defended the strikes, saying Iran had violated the ceasefire.

"I...