Iran has seized a tanker that appears to have been carrying its own oil, highlighting just how tangled its export system has become under sanctions and blockade. State media said Iranian forces detained the vessel, identified as the Ocean Koi, in the Gulf of Oman. The ship had already been sanctioned by the United States earlier this year as part of Iran’s shadow fleet, the network used to move crude outside official channels. Iran’s explanation didn’t add much clarity. Officials said the tanker...