ADEN (Reuters) - Warships shelled suspected al Qaeda strongholds in a mountainous region of southern Yemen on Thursday, government officials said.

The officials, who asked not to be named, said they believed U.S. forces carried out the operation, though Pentagon spokesman Captain Jeff Davis quickly denied any U.S. involvement.

The strikes come less than a week after a covert U.S. Navy SEAL raid, also in Yemen’s south, the first ordered by U.S. President Donald Trump as commander in chief.

The naval attacks appear to be part of an intensifying campaign against one of the most active branches of the Islamist militant network.

“Ships fired several missiles towards the al-Maraqisha mountains, where al Qaeda elements are based. The ships are widely believed to be Americans,” said one official, who asked not to be identified because of the sensitivity of the matter.

“We have received no information on the outcome of the shelling.”

The United States regularly uses drones to hunt suspected al Qaeda militants in Yemen’s barren hinterlands.

The al-Maraqisha mountains are a key al Qaeda stronghold in southern Yemen. Militants took refuge there last year after Yemeni government forces, backed by Arab coalition aircraft, drove them from the cities of Zinjibar and Jaar.

The militants have exploited a civil war between Iran-aligned Houthi rebels and forces loyal to the internationally-recognized President Abd-Rabbu Mansour Hadi to regroup in the impoverished country, which shares a border with Saudi Arabia.

Hadi’s forces have made gains against the Houthi rebels in past months, advancing north into al-Mokha and Dhubab last week in a bid to deprive the Houthis of strategic Red Sea ports.

In a separate incident, six soldiers loyal to Hadi were killed when a bomb, apparently planted by al Qaeda militants, exploded near the city of Lawdar in the Abyan province.