ISIS on Monday said its fighters carried out an attack on the northern Mozambique town of Palma, where dozens were killed, thousands displaced and some remain missing.
Insurgents hit the town, adjacent to gas projects worth $60 billion, with a three-pronged attack on Wednesday.
Fighting continued on Monday, a security source in the town said.
The government confirmed on Sunday that dozens of people had died, including seven when their convoy of cars was ambushed during an escape attempt.
ISIS claimed the attack through its Amaq news agency, saying its fighters had taken control of the town after days of clashes with security forces.
They killed at least 55 people, including soldiers, destroyed and took control of buildings including factories and banks, and seized vehicles, it said.
Most communications in Palma have been down since Wednesday, making the claims difficult to verify.
Officials at Mozambique's Defence Ministry and its national police could not be reached for comment.
The country's northern-most province of Cabo Delgado, where Palma is located on the border with Tanzania, has been home since 2017 to an insurgency now linked to ISIS.
The number of dead or missing after the attack remains unclear. Tens of thousands of people may have fled, aid workers said.
Nick Alexander, a British-South African contractor, was in a convoy of vehicles trying to escape a besieged hotel on Friday, his daughter Jayde said.
Mr Alexander spent two nights crawling in the bush after the ambush before being rescued, Jayde said.
She said he had grabbed a machinegun from a military vehicle for protection but it did not work.
"He is alive and well and on his way home," she said.
Many people fled to the beach in hopes of being rescued, another security source said.
Boats were taking them to the provincial capital, Pemba, about 240 kilometres to the south, an aid worker said.
Government-contracted helicopters have also been rescuing people, the security sources said.
Relatives of the missing waited anxiously at Pemba's port, hoping to hear news of their family members.
"I have had no contact with my family since Wednesday, my wife, my sons, my mother, my brothers," Pemba resident Patricio Amade told Portugal's state news agency Lusa.
Lusa reported on Monday that hundreds of people who fled Palma had arrived at the border with Tanzania after walking through the bush for days to reach safety.
Tanzania's police said they had not so far seen people seeking to enter their territory from Mozambique.
At least one South African was among those killed. Britain's The Times newspaper reported that a British national had died.
French energy group Total said on Saturday it was calling off a planned resumption of construction at its $20 billion development outside Palma after the attack, and would reduce its workforce to a "strict minimum".
The company pulled out most of its workforce in January because of security issues.