Story highlights More than 40 militants enter port city of Aden, threatening to free imprisoned fighters

"Our forces are ready and will ensure ... safety," a Defense Ministry official says

Hundreds of reinforcement troops are brought in to prepare for a possible break

More than 40 al Qaeda militants infiltrated Yemen's port city of Aden seeking to free imprisoned fighters, the defense ministry said Saturday.

The ministry said the militants entered through neighboring al-Thale province, and that the country is on high security alert.

"The same group of militants already attacked a government prison in al-Thale province seeking to release members of the terror network arrested in a military operation earlier this week," a senior Defense Ministry official told CNN on condition of anonymity because he is not authorized to talk to media.

The militants were heavily armed and attacked the governmental complex with rocket-propelled grenades and a barrage of bullets.

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"After failing to free prisoners in al-Thale, they headed towards Aden. Our forces are ready and will ensure the safety for Yemen's business capital," the official said.

Hundreds of reinforcement troops were deployed to Aden since Thursday.

Prison breakouts are not new for Yemen. Last week, 24 al Qaeda militants escaped a highly secured prison in the southwestern province of al-Hodieda, security officials in the province confirmed to CNN.

Meanwhile, the Interior Ministry said Saturday that its forces seized control of all government buildings in the town of Azzan, Shabwa. The town was the last al Qaeda stronghold in Yemen.

Al Qaeda took over numerous towns in southern Yemen last year, benefiting from the political instability in the country. The crisis resulted in a transfer-of-power deal forcing longtime President Ali Abdullah Saleh to step down from power.

Newly elected President Abdu Rabu Mansour Hadi intensified the fight against al Qaeda and vowed to continue the assault on militant hideouts until the group is defeated.

Over the past month, more than a dozen militants were arrested throughout Yemen, including prime suspects behind deadly suicide attacks against government forces.