Yemeni government troops launched a surprise attack in the south of the country to recapture an al-Qaida stronghold, killing 25 Islamist militants, an official said Friday.

The official said the army succeeded in regaining control over one district on the outskirts of Zinjibar, but the rest of the provincial capital was still in al-Qaida's hands.

The official spoke on condition of anonymity because he was not authorized to speak to reporters.

A Defense Ministry statement on Friday said the offensive that started two weeks ago around another city in the southern Abyan province, Lawder, has so far killed 250 al-Qaida militants. Also, 37 Yemeni soldiers have died, it said.

During a year of internal turmoil that eventually led to longtime Yemeni President Ali Abdullah Saleh's resignation, al-Qaida took advantage of a security vacuum to overrun parts of the south.

The U.S. believes al-Qaida's branch in Yemen is the most dangerous arm of the terror group because of its repeated attempts to carry out attacks in the U.S.

In recent weeks, the Yemeni military has been hitting the militants in ground and air operations, while al-Qaida has carried out some bloody surprise attacks of its own against government forces.

Yemen's government expressed determination.

"The war on terrorism will expand and reach all the terrorist elements; it will continue and will not stop until it curbs it and uproots it," a statement Friday from the Ministry of Interior read.

Under a power transfer deal brokered by Arab Gulf countries and backed by the United States, Saleh received immunity from prosecution in return for stepping down. Protesters have been on the streets ever since, rejecting the terms.

The new president, Abed Rabbo Mansour Hadi, has pledged to purge Saleh's loyalists and family members from top security and military posts, a step toward restructuring the army to enable it to effectively combat al-Qaida militants in the south. Hadi's decisions have met with stiff resistance from the Saleh's allies.

The battle in the south between Yemen's army and the al-Qaida branch is seen as a test of Hadi's ability to combat the Islamist militants.

The Yemen military offensive in the south appeared to be making gains.

On Friday, the Ministry of Defense said in a statement, "the heroes of the Armed Forces have achieved a great advancement toward Zinjibar," the capital of Abyan province, where al-Qaida has been dominant.

Ansar al-Sharia, an al-Qaida-linked group, confirmed in a statement Friday that its members have "encountered a massive offensive by Sanaa regime forces, but they have failed" to retake Zinjibar.

Elsewhere in Yemen, demonstrators repeated their long-standing demands against Saleh.

Tens of thousands of Yemenis rallied in the capital, Sanaa, and several other cities demanding trial of Saleh and his family for killing protesters during past year's uprising.

"The people want to prosecute the murderer," the protesters chanted, holding up composite pictures showing Saleh behind bars.

Saleh stepped down in February but remains in Yemen. Some charge he is still meddling in state affairs through relatives and cronies in senior positions.