A US aircraft carrier was steaming toward Yemeni waters on Monday, joining other American ships in the area to monitor Iranian vessels suspected of carrying weapons for Huthi rebels in the worsening conflict.

The convoy, seen as suspect, is comprised of nine military-type ships, including two patrol boats, a senior defense official told AFP, saying its exact destination was unknown.

"We believe these vessels may have arms and equipment on board. If they are delivered to Yemen, it will further destabilize" the country, a second defense official said.

But the official also stressed that it was still "premature to speculate."

Pentagon spokesman Colonel Steven Warren denied reports the US ships have orders to intercept Iranian vessels carrying weapons for the Huthis.

Several US officials suggested that it would instead likely be Saudi Arabia, Egypt and their allies that would intercept Iranian ships if needed.

"So far, we are doing a security mission; we are not part of the maritime interdiction mission," a military official told AFP.

One US official said the Iranian convoy had passed through the Strait of Hormuz to leave the Gulf and was now headed west, thus potentially toward Yemen.

Earlier this month, the UN Security Council passed a resolution slapping an arms embargo on the Huthi rebels.

The USS Theodore Roosevelt aircraft carrier, which was in the Gulf to launch strikes against the Islamic State group, earlier passed through the Strait of Hormuz headed for the Gulf of Aden and the southern Red Sea.

It is being escorted by the USS Normandy, a guided missile cruiser. The US Navy said the pair will "ensure the vital shipping lanes in the region remain open and safe."

Nine US ships are now near Yemen, where last month Saudi Arabia and a regional coalition launched air strikes targeting Huthi rebels.

The United States says it is not taking part directly in the Saudi-led strikes, but provides intelligence and logistical support.