An attack on the northern Mozambique town of Palma where dozens were killed and thousands displaced may show the increasing "brazenness" of Islamic State in the country, a U.S. official said on Monday.

The U.S. Department of State's acting special envoy for the global coalition to defeat ISIS, John Godfrey, told reporters the attack was in line with what has been seen from Islamic State in Mozambique and said the U.S. government is closely monitoring events. He cited what he said was the brutality of the attack and the killing of civilians as well as the increased brazenness of the group, which is now seeking to hold some towns.

"The attacks there are horrific, frankly, and show a complete disregard for the life, welfare, and security of the local population," Godfrey said of the attacks in Palma. "Attacks such as these are clear indicators that ISIS continues to actively seek to spread its malign activity to new fronts," he added.

Godfrey said one American was on the ground in Palma and that the understanding is the citizen has been successfully evacuated. Islamist insurgents hit the town, adjacent to gas projects worth $60 billion, with a three-pronged attack on Wednesday. Fighting continued on Monday, according to a security source directly involved in efforts to secure the town.

Islamic State claimed the attack via its Amaq news agency, saying its fighters had taken control of the town after days of clashes with security forces. They had killed at least 55 people, including a number of soldiers, destroyed and taken control of buildings including factories and banks, and seized vehicles, it said.

Reuters could not independently verify the claims. Most communications to Palma have been down since Wednesday.