In addition to conducting drone strikes, Americans flew Yemeni commandos to suspected al-Qaeda training camps where more than three dozen militant were killed over the weekend, CNN reported Tuesday.
U.S. personnel piloted Russian-made Yemeni helicopters to mask American involvement in the raids in the Mahfad mountains of southern Yemen, an unidentified official told the network.
Yemeni forces battled suspected members of al-Qaeda in the Arabian Peninsula, but no Americans were involved in the fighting, the official added.
CNN said the official did not say whether the U.S. personnel were special forces, CIA or private contractors.
DNA tests are being conducted to determine if senior commander Nasser al-Wuhayshi and master bomb maker Ibrahim al-Asiri were among the dead.
Early Saturday, at least nine suspected al-Qaeda militants and three civilians died in a U.S. drone strike on a convoy in al-Bayda province.
Pentagon spokesman Navy Rear Adm. John Kirby would not detail the U.S. role in the anti-terror operations.
"We continue to work with the Yemeni government and the Yemeni armed forces to help them improve their counterterrorism capabilities inside the country," Kirby told CNN. "That work continues, and it will continue."
In apparent retaliation for the weekend attacks, four senior Yemeni security officials -- three of them colonels -- were assassinated by gunmen on motorcycles over 24 hours, the Yemen Post reported Tuesday.
Three officers were killed in Sanaa, the capital, and a deputy security chief was gunned down in the central Yemeni district of Harib, the paper said.