Yemen's army seized the Al-Qaeda strongholds of Jaar and Zinjibar on Tuesday, officials said, more than a year after the jihadists captured most of Abyan province.

In the first major victories of a month-long offensive, troops backed by armoured vehicles stormed the town of Jaar after Al-Qaeda gunmen withdrew overnight, and hours later took control of the provincial capital of Zinjibar, officials and witnesses said.

Al-Qaeda militants had overrun most of the southern province of Abyan including Jaar and Zinjibar more than a year ago, taking advantage of a central government weakened by Arab Spring-inspired protests.

"With the cooperation of the citizens of Abyan... the heroes of the armed forces and the popular resistance committees have taken full control of the city of Jaar," the defence ministry said, quoting southern military commander Salem Ali Qoton.

"Al-Qaeda has suffered heavy losses... and dozens of militants have fled" the town, said Qoton, adding the army was able to reopen the main road linking Abyan province with the southern port city of Aden.

The army also took control of Zinjibar, 12 kilometres (7.5 miles) to the southeast of Jaar, a top military official said later.

"Zinjibar in total has fallen" in the hands of government forces, said General Mohammed al-Somali, the head of 25th Mechanised Brigade.

"Al-Qaeda fighters have fled the city after the noose was tightened on them," he said, adding the militants had planted landmines and explosives before running away.

Twenty Al-Qaeda fighters were killed and others wounded in Jaar, Qoton said, while the bodies of six militants were found in Zinjibar, according to another military official who did not elaborate whether they were killed in Tuesday's fighting.

As news of Al-Qaeda's ouster spread, dozens of Jaar residents took to the streets celebrating and firing guns into the air, witnesses said.

Armoured vehicles moved into the town centre in the morning, just hours after Al-Qaeda militants withdrew towards the nearby town of Shuqra, when "fighting with the army became fiercer," one resident said.

Witnesses said they saw vehicles carrying militants, weapons and furniture heading east towards Shuqra, where many Al-Qaeda leaders are believed to be holed up.

The militants distributed pamphlets in the town apologising to residents for dragging Jaar into a conflict with the army and for the damage caused by the fighting, according to locals.

Yemeni forces launched an offensive on May 12 aimed at reclaiming towns and cities lost to Al-Qaeda during the past year.

Western diplomats say US experts have been assisting the army in their fight with Al-Qaeda in the Arabian Peninsula, or AQAP, the terror network's Yemen branch which Washington considers to be its deadliest and most active.

In recent weeks, the United States has also escalated drone strikes against Al-Qaeda militants hiding in Yemen's mostly lawless southern and eastern provinces, killing several dozen of them.

In May, just days before the launch of the government offensive, a drone strike killed Al-Qaeda leader Fahd al-Quso who was wanted in connection with the 2000 bombing of the USS Cole.

Since the offensive began, 485 people have been killed, according to an AFP tally combined from different sources. This includes 368 Al-Qaeda militants, 72 soldiers, 26 local armed men and 19 civilians.