SANAA, Yemen – Yemen’s political upheaval has emboldened suspected al-Qaida militants who have seized a provincial capital and now are operating openly in the lawless south, training with live ammunition and controlling roads with checkpoints.

The U.S. fears the power vacuum will give even freer rein to al-Qaida’s branch in Yemen — already the terror network’s most active franchise.

Yemen’s government said Thursday that troops killed 12 suspected al-Qaida militants as part of a campaign to retake the southern city of Zinjibar.

Hundreds of militants seized the capital of troubled Abyan province on May 27, taking advantage of a breakdown of authority resulting from the government’s battle with armed tribesmen seeking to topple President Ali Abdullah Saleh, the country’s autocratic leader of more than three decades.

Yemen’s crisis has deepened further since Saleh was critically wounded in a June 3 rocket attack on his compound and flown to neighboring Saudi Arabia for urgent medical treatment. U.S. officials say the 69-year-old Saleh suffered burns over 40 percent of his body and has bleeding inside his skull.

The developments have fueled fears of growing instability in a nation that has been a launching pad for repeated attacks against the United States.

The Yemen-based al-Qaida in the Arabian Peninsula has been linked to several attempted attacks on U.S. targets, including the foiled Christmas Day 2009 bombing of an airliner over Detroit and explosives-laden parcels intercepted aboard cargo flights last year.

Yemen is also home to radical U.S.-born Muslim cleric Anwar al-Awlaki, whom Washington has put on a kill-or-capture list and accused of inspiring attacks on the U.S., including the 2009 shooting at Fort Hood, Texas, that killed 13 people.

In Abu Dhabi, U.S. Secretary of State Hillary Rodham Clinton called Thursday on all sides to honor a cease-fire. She said Washington was pushing for an “immediate, orderly and peaceful transition” in Yemen.