It was one of several attacks on the town - 60km (37 miles) South of the projects being developed by oil majors like France's Total - this year as fighters with links to the ISIL (ISIS or Daesh) armed group have stepped up attacks in the region. Its port is used for cargo deliveries to the developments.
"The port of Mocimboa da Praia was captured by the terrorists at dawn," the Moz24Horas website reported, while a military source told AFP news agency the small town and its port had "fallen".
Mozambique's defense forces (FDS) confirmed "terrorists" launched "sequenced attacks" on several villages surrounding the port over the past week in an attempt to occupy the town.
"At the moment, there are ongoing actions to neutralise the terrorists that are using populations in the affected areas as shields," the FDS announced in a statement.
Attacks in the Northern region have already killed at least 1,300 people, according to the Armed Conflict Location & Event Data Project (ACLED).
Displaced people number more than 250,000, according to aid organisations working locally.
In its latest weekly report released on Wednesday, ACLED said "insurgents and government security forces have been in more or less constant running battles in the area" for the past week.
The attacks started in 2017 in Mocimboa da Praia and have since spread to massive swaths of Cabo Delgado province.
The latest attack - the third on the town this year - was claimed by the Islamic State Central Africa Province (ISCAP).
The ISIL-affiliated group has the stated goal of establishing a caliphate in the region. ISCAP has claimed several attacks since June 2019 via social media, often posting images of slain soldiers and seized weapons.