LONDON, June 23 (Reuters) - An evacuation plan to enable hundreds of ships with some 11,000 seafarers stranded in the Gulf to sail through the Strait of Hormuz is underway after Iran and the U.S. reached a ceasefire deal, the United Nations' shipping agency said on Tuesday. "We have now started contacting the ships to start the evacuation," a spokesperson with the U.N.'s International Maritime Organization (IMO) said, without providing a timeframe.