Yemen's army, with the backing of US experts, is slowly gaining ground in its southern offensive against Al-Qaeda, diplomats and officials said on Wednesday, as the death toll in five days of fighting rose to 128.

A military official said the army on Wednesday had advanced towards the southeastern entrance of Zinjibar, which was captured by Al-Qaeda militants a year ago, following clashes that left six soldiers wounded.

"The army is deployed at the entrances of the city. It advances during the day and tactically retreats at night out of fear of terrorist attacks," an officer on the ground at Zinjibar, capital of southern Abyan province, said.

The military source said a force of around 20,000 soldiers from all regions in the south are engaged in the all-out operation launched on Saturday to retake Al-Qaeda strongholds in Abyan province.

They are backed by armed local militia consisting of residents of towns and cities determined to put Al-Qaeda to flight.

Another 13 people were killed in fighting on Wednesday, most of them jihadists, officials and tribal sources said.

According to Western diplomats in Sanaa, US experts have been deployed on ground, mainly at Al-Anad airbase in the nearby Lahij province, to offer advice to Yemeni troops.

At the same time, US drones are launching surgical strikes targeting the militants, according to witnesses, who have also reported that US naval units are bombing jihadists' positions from the sea.

AFP has not been able to confirm this information from official sources.

On Sunday, John Brennan, US President Barack Obama's top counter-terrorism aide held talks in Sanaa with Yemeni President Abdrabuh Mansur Hadi on "combatting terrorism" and attempts by Yemen's army to crush the local branch of Al-Qaeda, official media said.

The offensive was launched after newly-elected Hadi vowed to fight the network and followed days after the White House announced that a plot by Al-Qaeda in the Arabian Peninsula (AQAP) to blow up a US airliner had been successfully thwarted.

A senior US official told the New York Times that a bomb for the would-be attack was sewn into "custom fit" underwear that would have been difficult to detect even in a careful pat-down at an airport.

The paper said a double agent managed to spend weeks with AQAP before handing over information that allowed the United States to launch a drone strike in Yemen on May 6 that killed Fahd al-Quso, a senior figure who was wanted for the 2000 bombing of the USS Cole in Yemen.

Quso's name figured on an FBI list of most wanted terrorists, along with a reward of up to $5 million for information leading to his arrest.

On May 12, US drones launched another two air strikes, killing a total of 11 suspected Al-Qaeda jihadists in eastern Yemen.

On the ground, the army backed by local militiamen on Wednesday exchanged artillery fire with Al-Qaeda insurgents in an area surrounding Mount Yasuf, overlooking Loder, northeast of Zinjibar, witnesses said.

A member of the Popular Resistance Committees, a militia formed by local residents, said that Al-Qaeda militants had at dawn fired artillery shells at troops and militiamen stationed atop Mount Yasuf, killing two armed residents and wounding four.

The army responded and in a fierce counter-attack killed 11 jihadists, tribal sources said. The area then fell quiet, they added.

According to a military source, one soldier who had replaced Al-Qaeda's black flag with the Yemeni flag on the hill on Tuesday was shot dead by a jihadist sniper.

Saturday's multi-pronged assault is the largest conducted by the army against Al-Qaeda branch in Yemen, whose members have named themselves "Partisans of Sharia" (Islamic law).

At least 128 people -- 82 jihadists, 20 soldiers, 16 civilians, and 10 militiamen -- have been killed in the offensive, according to an AFP tally based on reports by officials and tribal leaders.

Al-Qaeda militants last year swept across southern Yemen, exploiting the decline in central government control that accompanied Arab Spring-inspired protests that eventually forced president Ali Abdullah Saleh to cede power in February.

Al-Qaeda chief Ayman al-Zawahiri exhorted Yemenis to rebel against Hadi, saying in a video posted on jihadist forums on Tuesday that he was an agent of the United States.

"You need to realise what is going on around you," Zawahiri said in his message, encouraging people to "rid the country of the corrupt politicians and vampires that suck the blood of the people."