Violence stoked by armed Islamists in gas-rich Cabo Delgado province has left at least 2,600 people dead since 2017, about half of them civilians, according to an NGO called the Armed Conflict Location and Event Data (ACLED).

Local people are "caught between the Mozambican security forces, the private militia fighting alongside the government and the armed opposition group locally known as 'Al-Shabaab' -- none of which respect their right to life, or the rules of war," said Amnesty's regional chief, Deprose Muchena.

"All three have committed war crimes, causing the deaths of hundreds of civilians."

Government officials contacted by AFP refused to comment and Interior Minister Amade Miquidade did not answer calls to his phone.

The authorities have previously denied their soldiers have committed any atrocities in Cabo Delgado.

Amnesty said its analysis was based on interviews with dozens of the nearly 670,000 internally displaced people, as well as reviews of videos and pictures, including satellite imagery.

The watchdog said the government hired a South African private military company, Dyck Advisory Group (DAG), after it "lost a number of battles" in its quest to regain control of the region.

The mercenaries have fired guns from helicopters and dropped hand grenades indiscriminately into crowds, according to Amnesty.

AFP made several requests to DAG for comment but did not immediately receive a reply.

Amnesty said it had probed an attack by helicopter at a hospital in the port town of Mocimboa da Praia last June, and schools had come under fire in other incidents.

It said it had verified video of another incident in September, in which government troops beat a naked woman with a wooden stick, shooting her 36 times and leaving her body on the highway.

- 'Summarily executed' -

In another gruesome killing, security forces blindfolded and shot several men in Quissanga before dumping their bodies in a mass grave, Amnesty said.

After that attack, "government security forces took women to be raped at the nearby base they had set up, where they also detained, beat, and summarily executed more men", it said.

The jihadists are also accused of heinous acts of violence with machetes, including numerous beheadings and desecration of corpses.

Concerns have grown that the use of foreign private militia in the conflict is exacerbating the crisis.

Jasmine Opperman, a security expert and analyst for ACLED, said DAG's deployment was a "desperate attempt to get air support" by the government, as well as an attempt to dodge accountability.

"They have transgressed the laws on many fronts," she said, urging Maputo to secure "direct support that is accountable at all costs".

International Crisis Group consultant in Southern Africa Piers Pigou called for "credible investigations" into the allegations of atrocities.

It was vital that "every effort is made to minimise possibilities of transgression," he added.