Rwandan and Mozambican police speak to returnees in Palma, Cabo Delgado province, Mozambique Sunday, Aug. 15, 2021. AP
The Thursday morning assault occurred at a facility belonging to India-headquartered Gemrock which neighbours Gemfields' Montepuez Ruby Mine (MRM).
Following the attack, Gemfields "has initiated the process of evacuating operational employees and contractors, and therefore mining operations at the site have ceased", it said in a statement.
No fatalities have been reported at the mine and "details of the attack are being verified", Gemfields added.
According to the precious gem miner, the military was being deployed to shore up police and mine security on the ground.
A deadly insurgency erupted in gas-rich northern Mozambique, near the Tanzanian border, five years ago -- concentrated mainly in and around coastal areas.
A regional military mission deployed last year had restored a sense of security, pushing militants from some of their traditional turf.
But the insurgent attacks are spilling inland.
Montepuez, which is situated inland, had largely been spared until recently.
A deadly March 2021 attack on the coastal city of Palma by fighters affiliated to the Islamic State group also put the brakes on a $20-billion gas project by French energy giant TotalEnergies.
The insurgency has claimed more than 4,300 lives and displaced some 820,000 people since the unrest erupted in October 2017.