SANAA, Yemen — Yemeni warplanes and troops backed by heavy artillery waged a four-front assault Tuesday against the strongholds of al-Qaida militants in the south, with U.S. troops for the first time helping direct the offensive from a nearby desert air base-turned-command center.

Yemeni military officials said dozens of U.S. troops were operating from al-Annad air base, about 45 miles from the main battle zones, coordinating assaults and airstrikes and providing information to Yemeni forces.

The officials said it was the most direct American involvement yet in the country's expanding campaign against al-Qaida's branch in Yemen, which has been blamed for directing a string of unsuccessful bomb plots on U.S. soil from its hideouts in the impoverished country at the tip of the Arabian Peninsula.

Most recently, this month it emerged that the CIA thwarted a plot to down a U.S.-bound airliner using a new, sophisticated explosive to be hidden in the bomber's underwear. But the planned bomber was actually a double agent who turned the device over to the U.S. government.

The offensive is the most concerted yet aiming to uproot al-Qaida militants who since last year have held a swath of territory, including the provincial capital Zinjibar and several other towns, in the south of the country. One Yemeni military official said the country's defense minister and an American general, whom he did not identify, were jointly overseeing the assault.

The Yemeni military officials, who are familiar with the workings of the army in the south, spoke on condition of anonymity to discuss the highly sensitive cooperation.