SANAA, Yemen -- Government warplanes and artillery struck Islamic militants who seized a town in southern Yemen, where extremists suspected of links to Al Qaeda have begun operating openly, training with weapons and controlling roads, emboldened by the country's political turmoil.

The U.S. fears that the impoverished country's power vacuum will give even freer rein to Al Qaeda's branch in Yemen -- already the terror network's most active franchise.

Early on in Yemen's upheaval, militants in March seized the small farming town of Jaar and surrounding areas. On May 27, thousands of militants took control of the nearby city of Zinjibar, capital of southern Abyan province on the Arabian Sea coast, 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 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.

Yemeni troops have struggled to retake the areas in the south. On Friday morning, warplanes hit militant positions north of Jaar, witnesses and security officials said. They said there were casualties but the number was not known. The night before, troops shelled other militant positions near the town with artillery, killing at least six militants, according to medical officials. The medical and security officials spoke on condition of anonymity because they were not authorized to talk to the press.

On Thursday, government troops advancing toward Zinjibar killed 12 militants outside the city, the Defense Ministry said.

The government says the militants are suspected of links to Al Qaeda, though their identity is unclear. Yemen is home to thousands of Islamic militants, many of them veterans of "jihads" in Afghanistan, Iraq and elsewhere, others homegrown extremists. Many have past links to Al Qaeda, though most are believed to operate independently. Al Qaeda's branch in the country is believed to number several hundred fighters.

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 Qaeda 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.

Saleh, who has clung to power in the face of massive street protests since February, had in recent weeks deployed his best armed and trained military units to fight tribesmen in the capital, Sanaa, who have joined in demands for an end to his regime.

While Saleh's departure for Saudi Arabia has led to a lull in fighting in the capital, it remains fraught with tension as troops led by Saleh's son and close relatives square off against the heavily armed tribesmen.

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.

The political impasse and commitment of elite forces to the fight in Sanaa has left Al Qaeda free to try to entrench its presence in a country that appears inching toward collapse. Emboldened, the militants have made inroads deep in the Yemeni hinterland and on its rugged mountain ranges.

In Abyan, residents said suspected Al Qaeda militants were openly training in camps and using live ammunition for target practice. They were also carrying out identity checks on travelers on roads leading to neighboring provinces.

"They have a great deal of influence and they use modern vehicles for transport as well as satellite telephones," said Abdullah al-Amari, an Abyan-based rights activist.

Residents of the southern province of Shabwa said suspected Al Qaeda militants and sympathizers had set up checkpoints on the road to the nearby province of Hadramawt. They also controlled the towns of Rawdah and Houtah, where they freely roamed the streets.

Shabwa-based army Lt. Col. Hassan Radwan said his unit knew the whereabouts of Al Qaeda fighters in the province as well as their training camps. "But when we tell our commanders, they tell us that they are just local tribesmen," he said.

At Hadramawt, a southern province on the Arabian Sea coast, activist Nasser Baqezqouz said the militants were so much at ease in the area that "they play dominoes in cafes without any fear."

Saleh's regime has long taken an ambivalent approach toward militants, making him a less-than-reliable partner in the U.S. fight against Al Qaeda.

There is a blurred line between Yemen's large and diverse community of militants and Al Qaeda, which is thought to have no more than 300 hard-core members in Yemen. The militants have varying levels of links to the terror network.

Saleh has allied with many of these groups to promote his own interests against political rivals that include moderate Islamists, leftist parties and secular-minded intellectuals. He has sought the militants' help to "Islamize" the south, where secular traditions endure two decades after it was united with the conservative north.

The upheaval of the past months has left Saleh too preoccupied to focus on the fight against Al Qaeda, and the United States has stepped up its covert operations in Yemen.

American officials said Thursday that a U.S. airstrike on June 3 killed a midlevel Al Qaeda operative named Abu Ali al-Harithia in southern Yemen.

A drone strike by U.S. special operations forces on May 5 targeted al-Awlaki, but a malfunction caused rockets to miss him by a matter of minutes, two American officials told The Associated Press. The officials spoke on condition of anonymity to discuss intelligence matters.

The recent U.S. operations in Yemen come after a nearly yearlong pause in American airstrikes, which were halted amid concerns that poor intelligence had led to bungled missions and civilian deaths that were undercutting the goals of the secret campaign.