Maersk said explosions nearby forced two ships operated by its US subsidiary and carrying US military supplies to turn around when they were transiting the Bab al-Mandab Strait off Yemen, accompanied by the US Navy.

"While en route, both ships reported seeing explosions close by and the US Navy accompaniment also intercepted multiple projectiles," Maersk said in a statement, adding it was suspending Red Sea transits by vessels of the US subsidiary.

A spokesperson for Yemen's Houthi military forces said they fired ballistic missiles at several US warships that were protecting two US commercial vessels.

Both commercial vessels are operated by Maersk Line, Limited (MLL), its US subsidiary that carries cargo for the Department of Defense, Department of State, USAID, and other US government agencies.

Both are enrolled in the Maritime Security Program (MSP) and Voluntary Intermodal Sealift Agreement (VISA) with the US government, which is why they were escorted through the strait by US Navy vessels, Maersk said.

MSP and VISA are programmes run by the US Defense Department to transport forces, supplies and equipment during times of war or national emergency.

The vessels and crew were unharmed and were being escorted back to the Gulf of Aden by the US Navy, Maersk said. Bab al-Mandab is the outlet of the Red Sea, between Yemen on the Arabian Peninsula, and Djibouti and Eritrea on the African coast.

The Houthi military spokesperson, Yahya Sarea, said in a statement that Houthi forces targeted a number of US warships with ballistic missiles on Wednesday in a "clash" that lasted more than two hours and led to a U.S. warship being directly hit and the two commercial vessels having "to withdraw and return."

"Several of our ballistic missiles struck their targets despite warships' attempts to intercept them," the statement said.

A US official said no ships were hit. "No U.S. warships or commercial vessels were struck by Houthi missiles today," said the official, who spoke on condition of anonymity.

Houthi forces fired three anti-ship ballistic missiles toward the U.S.-flagged container ship Maersk Detroit on Wednesday as it was transiting the Gulf of Aden, the U.S. Central Command said separately.

There were no reports of injuries or damage to the ship, it added in a statement.

'Escalation of risk'

On Jan. 5, Maersk said it would divert its container vessels away from the Red Sea for the foreseeable future. Until Wednesday, MLL ships were the exception, but that would no longer be the case, Maersk said.

"Following the escalation of risk, MLL is suspending transits in the region until further notice," it said on Wednesday.

Seafarers remain in the firing line, and have signed agreements to receive double pay when entering the high-risk zones.

"There's a feeling of vulnerability," Stephen Cotton, General Secretary of the International Transport Workers' Federation, the leading union for seafarers, told Reuters.

"The feedback from the (ship) captains is certainly on the container trades, they are much happier to go around the Cape."

US maritime labour organizations have been separately concerned about the impact on US flagged ships, which in recent weeks have been subjected to what nine unions called "the most significant attacks on the United States Merchant Marine in more than half a century".

"It is critically important that US-flag vessels carrying commercial, military and foreign aid cargoes are provided the necessary protection from the United States military as they transit the increasingly treacherous waters of the Red Sea," the unions wrote in a Jan. 19 letter to the US Transport Command.