The U.S. Navy accompanied four American-flagged ships and a British vessel moving through the Strait of Hormuz at the mouth of the Persian Gulf on Thursday, and officials said the U.S. will offer that aid to any other nation concerned about interference from Iranian vessels.

U.S. Army Col. Steve Warren, a Pentagon spokesman, said the four U.S. ships belonged to the Navy's Military Sealift Command or were contract vessels. Those ships have civilian crews and are used to carry cargo or re-supply U.S. Navy ships.

Air Force Col. Pat Ryder, a spokesman for U.S. Central Command, said that any U.S.-flagged ship can ask to be accompanied by Navy warships through the narrow strait, which includes Iranian territorial waters.

Officials use the word "accompany" rather than escort, because they say the Navy ships are positioned nearby and are ready to respond if needed, but they don't travel back and forth through the strait alongside each contract or Sealift Command vessel as they would during an escort. Ryder said the U.S. Navy ships don't intend to go into Iranian territorial waters.

The policy was adopted after Iran Revolutionary Guard Corps naval vessels reportedly fired warning shots near a Marshall Islands-flagged cargo ship earlier this week and have detained it and its crew.

Iran says it intervened with the Maersk Tigris because the Maersk shipping line owes it money awarded in a lawsuit.