In that case, the Navy said it spotted an Iranian Islamic Revolutionary Guard Corps naval support ship towing a Saildrone Explorer. The American seamen told the Iranians that the drone was U.S. government property and asked for it back. With a U.S. Fifth Fleet MH-60S Sea Hawk helicopter hovering above, the Iranians disconnected the towing line they had attached to that drone and eventually departed the area, according to a U.S. Navy release at the time. The Navy resumed operations "without incident," the release said. The United States and Iran often tangle in the Persian Gulf -- which the United States calls the Arabian Gulf -- and in the Red Sea and the Arabian Sea.

An Iranian ship seized an American military unmanned research vessel in the Gulf but released it after a US Navy patrol boat and helicopter were deployed to the location, the Pentagon said Tuesday. The US Central Command's 5th Fleet said a support ship from Iran's Islamic Revolutionary Guard Corps Navy, the Shahid Baziar, was spotted towing the seven-meter (23-foot) Saildrone Explorer unmanned surface vessel (USV) late Monday. The US naval drone, equipped with an array of sensors, radars and cameras, was in international waters collecting navigation and other unspecified data, the 5th Fleet said in a statement. When the Iranian vessel was seen towing the unmanned boat, US forces sent the USS Thunderbolt coastal patrol ship, which was operating nearby, to the scene. In addition, an MH-60S Sea Hawk helicopter based in Bahrain flew to the location.