Al-Qaeda militants have seized control of key areas in and around Yemen's port city of Aden, high-ranking security officials say, a major gain for the group which has been making inroads amid the chaos of the country's civil war.

The move, part of a weeks-long expansion in Aden in the wake of major fighting there between Shia rebels known as Houthis and pro-government forces backed by Saudi Arabia, shows how the organization still holds formidable clout despite its relatively low-profile role in the country's raging civil war.

Fighters took Tawahi district, home to a presidential palace and Aden's main port, and were patrolling the streets flying black banners, the officials said Saturday. The militants also took parts of Crater, Aden's commercial centre, and parts of the town of Dar Saad, just north of Aden, including an army base that their fighters turned into a training camp, they added.

Security officials near the seized base, in Dar Saad's al-Lohoum district, said it is now training an estimated 200 militants.

The officials, who hail from the military, security forces and police, all spoke on condition of anonymity because they weren't authorized to speak to journalists. Yemeni government spokesman Rajeh Badi, now based in Saudi Arabia, did not respond immediately to requests for comment.

Yemen's conflict pits the Iran-allied Houthi rebels and troops loyal to the former president, Ali Abdullah Saleh, against an array of forces including southern separatists, local and tribal militias, Sunni Islamic militants as well as troops loyal to President Abed Rabbo Mansour Hadi. The conflict gained international attention when the Houthis took over the capital, Sanaa, last September, and escalated in March as a Saudi-led coalition started launching airstrikes against Houthi positions.

Backed by heavy airstrikes, the coalition forces pushed the Houthis out of Aden last month, and have been pursuing them farther to the north. They had been making rapid advances in armored columns until earlier this week, when rebels ambushed them in a major battle near the entrance to Bayda province.

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A man walks on the rubble of his house, which was destroyed by Saudi-led air strikes on the nearby offices of the education ministry's workers union in Yemen's northwestern city of Amran on Aug. 19. (Khaled Abdullah/Reuters)

Al-Qaeda recruitment drive 

Al-Qaeda appears to have taken advantage of a security vacuum in Aden to ramp up its presence. It also has forces in the cities of Breiqa, west of Aden, and al-Khadra, the officials added.

Omar al-Sobeihy, a resident of Dar Saad, said that while al-Qaeda fighters are moving freely in the area, "we haven't felt any harassment from them so far."

In the Tawahi district, resident Taha Faris described a similar situation.

"They are spread out in Tahawi and we can say they have the area under their control. So far they aren't harassing people, trying instead to gain support, though I fully realize they are waiting for the right moment to attack and control all of Aden," Faris said.

Washington considers al-Qaeda's Yemen branch to be the most dangerous offshoot of the terror network.

In Tawahi, al-Qaeda destroyed the main state security building on Saturday with a powerful bomb that was heard around the city, security officials said. The group has been trying to attack the site for several years, they added. Meanwhile, an official with port security said one of their boats was set ablaze by suspected al-Qaida militants.

Al-Qaeda, which had only a minor role the war against the Houthis, also has been on a major recruitment drive, adding hundreds of young men to its ranks and stockpiling weapons, several officials said.

Drone strikes

The United States has kept up its drone attacks targeting the militants, including one in June in the city of Mukalla that killed the group's top leader. Al-Qaeda has been in control of Mukalla, the capital of the Hadramawt province, which borders Saudi Arabia, since April.

Al-Qaeda and ISIS members were also present in al-Houta, capital of Lahj province, according to witnesses and security officials.

A group of activists in Aden warned Hadi in an Aug. 11 letter that the town of al-Sheikh Othman in Aden province had become a centre for arms dealers, voicing their frustration over the security vacuum. The well-known activists, who campaign for more autonomy for the south, said al-Qaeda and other sleeper cells were buying and storing weapons from that market.

A high-ranking military official said the authorities in Aden wrongfully gave weapons to al-Qaeda when they randomly distributed them to pro-government forces in Aden in March and April while pursuing the Houthis. Al-Qaida captured other weapons in clashes, according to the official.