The Maersk Tigris, a commercial shipping vessel sailing under the Marshall Islands flag, was fired upon and seized today by the Iranian Navy while sailing on a known commercial shipping route through the Strait of Hormuz.

Col. Steve Warren, a Pentagon spokesman, said the cargo ship’s master had initially refused an Iranian order to move further into Iranian waters, but after the warning shots were fired the Maersk Tigris complied.

The cargo ship, which had more than 30 people aboard, was directed to waters near Larak Island, he said.

The Iranian vessels, numbering five or six, were with the Iranian Revolutionary Guard Corps Navy, Warren said.

“It is inappropriate” for the Iranians to have fired warning shots across the ship’s bridge in those circumstances, Warren said, adding that it was too early to know whether the Iranian intervention amounted to a violation of the freedom of navigation through a waterway heavily used by international shipping.

The spokesman said the U.S. government has “certain obligations” to defend the interests of the Marshall Islands, but he was uncertain how those obligations apply in this situation.