SANAA, Yemen -- A Yemeni air force jet mistakenly bombed a bus in a southern city controlled by Islamic militants and suspected Al Qaeda members on Wednesday, killing four people, while clashes between militants and soldiers on the ground left 23 dead on both sides.

The bombing came as part of daily government attacks on militants who last month seized Zinjibar, the provincial capital of Abyan, security officials said. They said 12 people were also wounded in the airstrike that mistakenly hit a passenger bus.

Militants have lately taken advantage of the turmoil that has gripped Yemen since a popular uprising against President Ali Abdullah Saleh's rule began in mid-February inspired by Arab uprisings elsewhere, and have taken over several towns in the south, far from the reach of the central government in the capital, Sanaa.

Shortly after Wednesday's airstrike, militants on the ground in Zinjibar overran a football stadium on the city's outskirts and attacked government troops there, the officials said. The clashes killed 15 security troops while eight militants died when government warplanes were called in and bombed the stadium.

Several government armored vehicles were destroyed and officials said scores of militants were wounded in the battle.

Also in Zinjibar, officials said militants seized 50 residents, accused them of passing information to the government, and locked them inside the governor's office.

Mohammed al-Tumeisy, one of those seized and later released, said his captors had warned he would be executed if he made any contact with the government.

The officials also said government jets on Wednesday bombed the nearby Jaar town, which was captured by militants in early April. And in the southern port city of Aden, a roadside bomb killed an army colonel late Tuesday.

All the officials giving the accounts of the fighting and casualties spoke on condition of anonymity because they were not allowed to talk to the media.

Residents in the south have criticized government forces for showing little appetite to face down the Islamist extremists. The militants in Zinjibar and now are operating openly there, training with live ammunition and controlling roads with checkpoints.

Their gains have fueled fears that they are successfully exploiting the power vacuum amid the uprising against Saleh. The revolt gained momentum when a coterie of the president's close aides, military commanders and Cabinet ministers joined the protesters.

Meanwhile, in the southern city of Taiz, activist Bushra al-Muktari said the Republican Guards shelled early Wednesday anti-Saleh protesters camped out at a central square, killing one and wounding four demonstrators.

Khaled al-Shayef, acting governor of Aden, was reported to have defected to Jordan, officials in the governor's office said Wednesday.

Saleh, who has ruled Yemen for nearly 33 years, himself left for neighboring Saudi Arabia on June 5 to treat severe wounds he suffered when his compound in the capital Sanaa was attacked.

It is not clear when -- or if -- he will return, deepening uncertainty in the poor nation at the southern corner of the Arabian Peninsula.