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/ Source: NBC News

Secretary of Defense Ash Carter has authorized the U.S. Navy to accompany all U.S.-flagged ships through the Strait of Hormuz into and out of the Persian Gulf, effective Thursday.

Pentagon spokesman Col. Steve Warren said the decision is a direct result of the Iranian Revolutionary Guard Navy's recently taking control of the Maersk Tigris and the harassment of the Maersk Kensington.

Thursday, U.S. Military Sealift Command and U.S. government contract vessels accompanied four ships through the Strait of Hormuz — three inbound and one outbound. American-flagged ships may request this accompaniment, but they are not obligated to use it. The four ships transited the strait without incident, Warren said.

Warren said this practice will continue "for an indefinite period of time."

Iran detained the Tigris, which carries the flag of the Marshall Islands, on Tuesday in the strait. On April 24, four Iranian gunboats "harassed" the American-flagged Kensington as it transited the strait into the Persian Gulf.

The Tigris remains anchored in Iranian territorial waters, near Larak Island, and has not let the 24 crew members leave.

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— Jim Miklaszewski and Courtney Kube