The US is providing military support, intelligence, ships and special operations forces to help in operations against al Qaida in Yemen, officials said.
The US military is helping Yemeni, Emirati and Arab coalition forces battling al Qaida in the Arabian Peninsula (AQAP), and recently retook the port city of Mukalla from the militants.
A senior US official said American special operations forces are advising Yemeni and Emirati forces in the region, but are working at headquarters level and are not near the conflict.
Navy Captain Jeff Davis, a Pentagon spokesman, said the US is providing "limited support" to the Arab coalition and Yemeni operations in and around Mukalla. He said that includes planning, airborne surveillance, intelligence gathering, medical support, refuelling and maritime interdiction.
Capt Davis declined to discuss whether special operations forces were in the country, but he said the US had sent a number of ships to the region including the USS Boxer amphibious ready group and the 13th Marine Expeditionary Unit. The USS Gravely and USS Gonzalez, both Navy destroyers, are also in the area.
"Trained and supported by an Arab coalition led by Saudi Arabia and the UAE, Yemeni government forces and resistance fighters have retaken Mukalla and continue their offensive against AQAP in eastern Yemen," said Capt Davis.
"AQAP remains a significant security threat to the United States and to our regional partners and we welcome this effort to specifically remove AQAP from Mukalla and to degrade, disrupt and destroy AQAP in Yemen."
Late last month forces loyal to Yemen's internationally recognised government drove AQAP militants from Mukalla, a year after they captured it. Mukalla had been their stronghold.
Al Qaida had gained ground amid the chaos that has reigned in Yemen since 2014, with Shiite Houthi rebels fighting the internationally backed government and its allies with the Saudi-led coalition.
Security officials and witnesses said al Qaida militants were also pulling out of Zinjibar and Jaar, two coastal cities east of the key southern port of Aden, following tribal-led negotiations.