People inspect damage at a site hit by Saudi-led airstrikes in the al Qaeda-held port city of Mukalla in southern Yemen. (Reuters)

Signaling a major shift in Yemen’s grinding civil war, Saudi-backed forces Sunday appeared to mount a large-scale offensive to drive militants aligned with al-Qaeda out of their strongholds in the country’s south.

The coordinated attacks on strongholds held by al-Qaeda in the Arabian Peninsula, known as AQAP, would be a first for the coalition dominated by Saudi Arabia, which began launching airstrikes and then ground attacks last year against Shiite rebels, known as Houthis. Saudi Arabia views the Houthis as proxies of its primary regional rival, Shiite Iran, and has refrained from targeting AQAP.

Under cover of airstrikes by the coalition, fighters aligned with Yemen’s internationally recognized government pushed toward the city of Mukalla and surrounding areas. Mukalla has become AQAP’s de facto capital.

Considered al-Qaeda’s most powerful franchise, AQAP seized the port city last year amid widening chaos in Yemen, a development that alarmed the United States. AQAP militants have seized much of Yemen’s southeastern coastal areas and used Mukalla as a base to plant even deeper roots in the impoverished Arabian Peninsula nation, possibly to stage attacks on Western targets.

Residents of Mukalla said coalition ground troops, including forces from the United Arab Emirates, entered the city from the east as AQAP militants fled.

“Coalition warplanes have been targeting the city since midnight, hitting the airport and cultural center,” said Ali Atkhaleqi, a freelance journalist who spoke by telephone from Mukalla.

Until now, coalition forces — including fighters linked to the government of Yemeni President Abed Rabbo Mansour Hadi — had not targeted AQAP. The terrorist group’s members also have fought the Houthis and, in turn, refrained from attacks against forces aligned with the coalition, raising suspicion of an informal truce between them.

The offensive targeting Mukalla and nearby areas comes amid recently restarted talks brokered by the United Nations to end Yemen’s multifaceted civil war. The fighting intensified last year after Houthi rebels toppled the government and advanced to the major southern port city of Aden, battling Islamist militants, southern separatists and Hadi allies along the way.

The peace negotiations in Kuwait City resumed Thursday with the major parties to the conflict in attendance, including representatives of former Yemeni president Ali Abdullah Saleh. His loyalist forces — many drawn from Yemen’s splintered military — have been crucial allies of the Houthis in battles against fighters aligned with the internationally recognized government.

The conflict has killed an estimated 6,000 people and created what U.N. officials describe as a humanitarian catastrophe. Previous U.N.-backed peace talks collapsed because of an inability to stem fighting on the ground.

Footage posted on social media, which could not be independently verified, purported to show clashes on the outskirts of Mukalla, as well as the aftermath of air raids on the city. In one video, heavy gunfire could be heard in the background as residents fled to safety in their vehicles.

Speaking by telephone from Mukalla, a journalist said many AQAP militants had withdrawn from the city by Sunday afternoon. Coalition fighters moved in from the east, he said, speaking on the condition of anonymity because of concerns for his safety.

“There is a lot of fear among residents as AQAP fighters flee,” he said. “People are relieved that they are going, but they fear that there will be chaos as a result.”

It is unclear why the coalition has turned its firepower on AQAP-held areas, although the offensive comes just days after President Obama visited Saudi Arabia. In private, U.S. officials have expressed concern that Saudi Arabia’s intervention in Yemen has empowered the al-Qaeda franchise.

Even as the Saudi-led coalition had avoided attacking AQAP until now, U.S. drone strikes have continued to target the group, killing senior leaders.

During Sunday’s assault on Mukalla, AQAP fighters appeared to put up little resistance. Unconfirmed reports on Twitter from residents in Yemen alleged that ahead of the offensive, local tribesman attempted to negotiate the withdrawal of AQAP fighters from the city.

Yazeed al-Jabari, a businessman in Mukalla, said friends described seeing columns of artillery-loaded tanks and trucks coming down from the Saudi border in the past 24 hours. UAE flags were spotted on the machinery, he said.

“All of this makes it clear to me that the coalition is intent on taking over the city,” Jabari said by telephone.

Troops from the United Arab Emirates provided critical momentum to allied Yemeni fighters during an assault last summer on Aden. Houthi militants were driven out, and Aden became a base for pro-coalition forces and Hadi’s government.

But Aden eventually descended into lawlessness, including attacks claimed by Yemen’s nascent Islamic State affiliate. One of those attacks, a car bombing, killed Aden’s governor in December.

Since the summer, the Saudi-led coalition’s progress has mostly stalled in the face of fierce Houthi counterattacks.