A flotilla of nine Iranian military and cargo ships that US officials feared was carrying arms to strife-torn Yemen sailed northeast in the direction of Iran on Friday, a move the Pentagon said helped to ease US concerns.

“The Iranian ships have turned around ... Obviously what their onward plans are, we don't know,” Defense Secretary Ash Carter told a small group of journalists traveling with him after a trip to California.

“It is a welcome event because it does contribute to de-escalation and that's what we're trying to suggest to all the parties there, is the best course, and those parties include the Iranians,” Carter said.

Army Colonel Steve Warren, a Pentagon spokesman, said earlier the flotilla was in international waters about midway along the coast of Oman on Friday and still headed northeast.

Warren declined to say the ships were going back to Iran or headed toward Iran. Warren said the US military did not know their intent and the vessels could turn around at any point.

President Barack Obama said on Tuesday the US government had warned Iran not to send weapons to Yemen that could be used to threaten shipping traffic in the Gulf. Carter said on Wednesday the United States was concerned the ships might be carrying advanced weapons to Houthi rebels there.

The US Navy sent the USS Theodore Roosevelt and an escort warship into the Arabian Sea earlier this week to support seven US warships already in the area around the Gulf of Aden because of concerns about growing instability in Yemen.

The Iranian-backed Houthis sidelined the Yemeni central government after seizing the capital Sanaa in September. The Houthis have continued to advance south capturing more territory.

A Saudi-led coalition, supported by the United States, launched an air campaign to destroy heavy weapons controlled by the Houthis that could threaten Saudi Arabia. The Saudis say their aim is to restore President Abd-Rabbu Mansour Hadi.

Saudi Arabia said on Tuesday it was ending its month-long bombing campaign against the Yemeni rebels and shifting to a new phase of its operations in Yemen. Bombing has continued since then, and US officials have said the Saudis had indicated they would continue to bomb as deemed necessary.