The Pentagon said on Tuesday that two Iranian warships have "linked up" with a cargo ship that Iran has said is carrying humanitarian aid to Yemen, as activists on board the vessel said it was due to arrive at the Yemeni port of Hodaida on Thursday.

"We're not overly concerned at this point," Pentagon spokesman Colonel Steve Warren told a briefing, adding that the ships' locations were being monitored "every step of the way."

Warren said the warships "linked up" with the Iranian cargo ship as it passed an area where the Iranian warships are conducting counter-piracy operations.

It was not immediately clear whether the warships were now escorting the cargo ship, or in close proximity due to the Iranian military exercises.

A coalition of Gulf Arab countries led by Saudi Arabia has imposed searches on all ships trying to enter Yemen in a bid to prevent weapons being smuggled to the Iran-allied Houthi group which controls much of the country, including Hodaida.

Saudi Arabia accuses Iran of arming the Houthis - a charge Tehran denies - while Iran has condemned the Saudi-led air strikes in Yemen. Iranian officials have said they would not let Saudi-led forces inspect the cargo ship, named Iran Shahed.

Two foreign nationals on board the vessel, who spoke before the Pentagon's comments, told Reuters the ship was not being accompanied by Iranian warships.

"It is a purely humanitarian mission. There is no ship accompanying us - let alone any Iranian warships," said Christoph Horstel, a German political activist aboard the cargo vessel.

"As I look at the horizon, there is no ship at all and during the whole trip there was never any warship," said Horstel, who previously worked for Germany's TV ARD.

Horstel said, however, that an unidentified plane had circled the ship three times on Monday.

"It is smooth sailing conditions and the expected time of arrival will be Thursday," he added.

Caleb Maupin, a native of Ohio who has campaigned against war and the U.S. financial system and is also aboard the Iran Shahed, also said he had not seen any escort warship.

Iran's Tasnim news agency on Monday quoted the Iran Shahed's captain as saying two Iranian warships had begun escorting the ship.

Deputy Chief of Iran's Armed Forces Brigadier General Massoud Jazayeri said last week that any attack on the ship would spark a regional war.

By Reuters