Two Iranian warships – an aging frigate and a helicopter assault ship -- have joined a convoy of Iranian cargo vessels in waters off Yemen that are being tracked by a U.S. flotilla including the aircraft carrier Theodore Roosevelt.

The Pentagon confirmed Thursday that two Iranian Navy ships were now escorting nine Iranian cargo vessels suspected of carrying arms for the Houthi rebels, who reportedly were still threatening the port city of Aden.

Pentagon spokesman Col. Steve Warren said the Iranian convoy was in the area where the Gulf of Aden meets the Arabian Sea but he could not confirm reports that the convoy appeared headed north back to Iran.

The Iranian ships "remain in the region," Warren said, and "they have not announced their intentions." The U.S. carrier Roosevelt was about 200 miles west of the Iranian convoy, Warren said, but the carrier's planes and other U.S. ships have monitored the convoy's movements.

"We don't know for certain what's aboard those ships," Warren said. On Wednesday, Defense Secretary Ashton Carter said the U.S. was concerned the ships might be carrying advanced weapons and he called on Iran to stop "fanning the flames" in the region.

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Navy ships in the area routinely board merchant ships to search for arms and contraband but only with the permission of the ship being boarded, Warren said.

"We have the authority now to do consensual searches – the key point they are consensual," Warren said.

If the Iranian ships were to cross from international into Yemeni territorial waters, it was expected that any searches would be conducted by Saudi Arabian and Egyptian Navy ships, which were also in the area.

Other nations have already evacuated their citizens from Yemen, but an undetermined number of Americans were still in the country. Warren said that "we are prepared to assist" in evacuations, but the military has yet to be given that task by the State Department or the White House.

The two Iranian warships were identified by Iranian officials earlier this week as the Alborz, an early 1970s-vintage British-made frigate that was sold to pre-revolutionary Iran, and the helicopter assault ship Bushehr, according to Iran's Fars news agency.

The two warships will "protect (Iran's) cargo ships and oil tankers against pirates," Iranian Navy Commander Rear Adm. Habibollah Sayyari said last weekend, according to Fars.

"There is no need for our flotilla of warships to dock in Yemen, but we are present in the Gulf of Aden powerfully" and "no one can warn the Iranian warships" to leave, Sayyari said.

On Tuesday, Saudi Arabia called an end to the air campaign against the Houthis in a bid for a ceasefire and political talks, but airstrikes continued Wednesday in response to what the Saudis said was continued advances by the Houthis.

At the Pentagon, Warren said that the Houthis appeared to be pressing the port city of Aden from three sides.

-- Richard Sisk can be reached at richard.sisk@military.com