SANA, Yemen - Yemeni troops backed by armed tribesmen routed Al Qaeda on Tuesday from two southern strongholds the terror network had held for more than a year, the most significant victory so far in a monthlong offensive against a local franchise that has tried repeatedly to bomb US-bound planes.

The military campaign, orchestrated by US military advisers and bankrolled by neighboring Saudi Arabia, has left Al Qaeda’s dangerous Yemen branch on the run. The group remained in control of only a handful of towns, with hundreds of its members scattered in the mountains, valleys, and vast desert of the Arab world’s most impoverished country.

In one of the liberated strongholds, Jaar, residents flocked to the town center, firing guns in the air in celebration after the army’s dawn attack. Others looted warehouses filled with humanitarian supplies delivered by relief groups, Waleed Mohammed, a resident, said in a telephone interview.

“We thought it would take a year for the army to get rid of Al Qaeda, but we were surprised when they swept into the town in no time,’’ said another Jaar resident, Khaled Mohsen. “I had been hearing a constant exchange of gunfire all night, then suddenly everything was quiet. I looked from the windows, and I saw soldiers in uniform in the center of the town.’’

The breakthrough follows other key victories against the Pakistan-based terror network since the death last year of Osama bin Laden.

Most notably, CIA drone strikes killed Abu Yahya al-Libi, Al Qaeda’s number two, on June 4 in Pakistan. Anwar al-Awlaki, an enormously influential American-Yemeni cleric, was killed on Sept. 30 in Yemen.

With the capture of Jaar and the city of Zinjibar, Yemen’s new US-backed leadership now has to deal with another fight in its war against Al Qaeda: sleeper cells, which are hard to chase.

“This is the end of Al Qaeda’s aspirations to establish an Islamic rule in the south. There is no comeback to this,’’ said Brigadier General Mohammed al-Sawmali.

“However, this is not the end of Al Qaeda in the country,’’ Sawmali said.

The militant group said it retreated to “spare bloodshed,’’ threatening to retaliate by attacking Yemen’s capital, Sana. In an e-mailed statement, the group addressed the Yemeni leadership as “crusaders and American agents’’ and warned, “we will chase you in your cities and palaces.’’

Some 500 Al Qaeda militants, including foreigners, fled Jaar after spray-painting walls and store shutters with slogans in red saying, “Al Qaeda has withdrawn. Al Qaeda was not defeated,’’ according to witnesses and officials.

The United States considers Al Qaeda’s Yemen branch to be the terror network’s most dangerous offshoot.

The group took advantage of a security vacuum last year amid a popular uprising against Yemen’s longtime leader, Ali Abdullah Saleh, to seize major population centers in Abyan Province. That raised fears it could use the area as a foothold to launch more attacks on US targets.

Already, the franchise has been blamed for directing a string of unsuccessful bomb plots on US soil from its hideouts.

The militants pose a threat to Yemen’s government as well and have looted army barracks, seizing tanks and heavy weaponry.

Al Qaeda claimed responsibility for a devastating suicide bombing last month by a soldier who blew himself up among troops rehearsing for a military parade in Sana, killing nearly 100 of them.

Tuesday’s success capped weeks of fighting as Yemen’s new president, Abed Rabbo Mansour Hadi, has pledged to uproot Al Qaeda from the south with help from the United States as part of a new cooperation following Saleh’s ouster.

The United States is helping the Yemenis form a command center staffed by dozens of US troops in the al-Annad air base in the southern desert, about 45 miles from the main battle zones. The Americans are coordinating assaults and airstrikes, and providing information to Yemeni forces.

While the United States has sent advisers and provided intelligence and support, Saudi Arabia, the oil-rich Gulf country, has come forth with cash - especially to armed civilians who back up the Yemeni army in its battles against Al Qaeda, according to a top military official.