Iran's Revolutionary Guards "harassed" a US-flagged commercial ship last week in the Gulf, days before seizing control of another cargo vessel in the Strait of Hormuz, the Pentagon said Wednesday.

The two incidents over a five-day period raised concerns about the security of shipping lanes in the strategic Strait of Hormuz, spokesman Colonel Steven Warren told reporters.

"It's difficult to know exactly why the Iranians are behaving this way," Warren said.

"We call on them to respect all of the internationally established rules of freedom of navigation, the law of the sea to which they are a signatory and other established protocols," he said.

In the April 24 incident, four Iranian patrol boats with the country's elite Revolutionary Guards "harassed a US-flagged merchant ship called the Maersk Kensington" as it moved along an established shipping route, he said.

The Iranian vessels "followed her for approximately 15 to 20 minutes in actions the ship master of the Kensington interpreted as aggressive," he said.

The American military was not involved in the incident and the Kensington's captain later filed a report with the US Navy describing the event.

On Tuesday, Iranian boats forced a Marshall Islands-flagged ship, the Maersk Tigris, to Iran's Larak Island after firing warning shots across the bow and boarding the vessel.

Iran has said it seized control of the container ship due to a commercial dispute with Denmark's giant Maersk group, which chartered the vessel to ferry cargo in the region. When it was intercepted, the Tigris was traveling on an international shipping route within Iran's territorial waters.

Iran's Foreign Minister Mohammad Javad Zarif, speaking at New York University, said a lawsuit was filed against the ship's owners between 15 and 16 years ago over undelivered cargo.

'Keeping an eye on things'
The US Navy sent a guided-missile destroyer, the USS Farragut, and a P-3 surveillance aircraft to the area to monitor the Maersk Tigris.

"What they are doing is keeping an eye on things. They are in close enough proximity to the Maersk Tigris that they will be able to respond if a response is required," Warren said.

As the US government had defense and other ties with the Marshall Islands, US officials were "in discussion with the Marshall Islands on the way ahead," he said, without elaborating.

But US officials said privately no military action was imminent.

The maritime incidents coincided with rising tensions in the region over the conflict in Yemen, with a Saudi-led coalition carrying out bombing raids on Iranian-backed Huthi rebels.

The United States is providing intelligence and other logistical help to the Saudis and their partners in the coalition.

Danish shipping group A.P. Moeller-Maersk said Wednesday that the crew of the Tigris cargo vessel was "safe."

The vessel was operated by Rickmers Ship Management, with head offices in Singapore and Hamburg, and was carrying cargo for the Maersk Line, the Danish group's shipping unit which had chartered the ship.