TEHRAN, Iran—The largest warship in the Iranian navy caught fire and later sank Wednesday in the Gulf of Oman under unclear circumstances, the latest calamity to strike one of the country’s vessels in recent years amid tensions with the West. The blaze began around 2:25 a.m. and firefighters tried to contain it, the Fars news agency reported, but their efforts failed to save the 207-meter (679-foot) Kharg, which was used to resupply other ships in the fleet at sea and conduct training exercises. State media reported 400 troops on board fled the vessel, with some 20 suffering injuries. The vessel sank near the Iranian port of Jask, some 1,270 kilometers (790 miles) southeast of Tehran on the Gulf of Oman near the Strait of Hormuz—the narrow mouth of the Persian Gulf. Satellite photos from Planet Labs Inc. analyzed by The Associated Press showed the Kharg off Jask with no sign …