A simmering Islamist rebellion in a remote corner of Mozambique has erupted into open warfare in recent weeks, with reports of massacres, beheadings and the brief seizure of two towns in the northern province of Cabo Delgado, writes BBC Africa correspondent Andrew Harding.
The armed men walked calmly through the long grass, skirting past a large white building, seemingly untroubled by the sound of gunfire.
Most carried automatic rifles and wore variations of what appeared to be Mozambican army uniforms. A few more shots rang out in the distance and someone shouted "Allahu Akbar" - God is the greatest - as if in reply.
The video footage, shot last month on a mobile phone in Muidumbe was powerful new evidence that a murky conflict in the northern-most region of Mozambique has now moved out into the open, in spectacular and alarming fashion.
A second video, shot a few weeks earlier, showed a dead man - apparently a policeman - lying in a pool of blood. The camera then moved over to reveal another corpse, then a third lying under a black police vehicle, then a fourth body out in the open, and finally a large pile of automatic weapons in some sort of police or military store.
How close are the links to Islamic State?
That footage was filmed in the strategic port of Mocimboa da Praia, which was briefly - and dramatically - seized by the militants on 24 March. Two days later, they seized another important town, Quissanga.
"Now they have guns and vehicles, so they move easily and can attack widely. And they are using soldiers' uniforms. So, people are very confused, and very afraid," said the Catholic Bishop of Pemba, Luiz Fernando Lisboa.
Those two large-scale, sophisticated military assaults are proof of a radical change in strategy for the group known locally as al-Shabab, although it has no known links to the Somali jihadi group of the same name, which is affiliated to al-Qaeda.
It has spent the past two years operating in the shadows, attacking remote villages across the province, ambushing army patrols on isolated roads, instilling terror in many rural communities, forcing perhaps 200,000 people to flee from their homes, but rarely giving any indication about its motives, its leadership, or its demands.
The video footage from both Mocimboa da Praia and Muidumbe district was quickly incorporated into the so-called Islamic State (IS) group's propaganda films, aired by the Amaq News Agency.
IS has claimed responsibility for a string of recent attacks in Mozambique, which has a Muslim population of about 18%, and appears to be promoting its involvement there as part of a "franchise" operation that has seen it expanding its footprint in several parts of Africa.
The idea that the rebellion in Cabo Delgado is, at its core, part of a global jihadist movement, has been given credibility by the militants themselves, who publicly swore allegiance to IS last year.
The relationship offers advantages to both sides.
But in a separate video, filmed this year and circulated widely on WhatsApp in Mozambique, a militant leader offered a much more nuanced explanation for the group's actions.
Locals complain about discrimination
"We occupy [the towns] to show that the government of the day is unfair. It humiliates the poor and gives the profit to the bosses," said the tall, unmasked man, in khaki uniform, surrounded by other fighters.
The man spoke frequently about Islam, and his desire for an "Islamic government, not a government of unbelievers", but he also cited alleged abuses by Mozambique's military, and repeatedly complained that the government was "unfair".
Observers say the evolution of the insurgency in Mozambique is remarkably similar to Boko Haram's emergence in northern Nigeria, with a marginalised group exploiting local grievances, terrorising many communities, but also offering an alternative path for unemployed youths frustrated by a corrupt, neglectful and heavy-handed state.
"It's very significant," said Eric Morier-Genoud, a Belfast-based academic and expert on Mozambique, of the militant leader's statement.
"He explains that he's a local, from Mozambique. He responds to the argument that they're all foreigners and denies it, and he denounces the present state as unfair and illegitimate," said Mr Morier-Genoud, arguing that the fact that most of the faces in the video are unmasked reveals "a clear gain of confidence".
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"For the first time, they spoke to the public," said Mozambican historian Professor Yussuf Adam, who said that the video gave further weight to the argument that the conflict in Cabo Delgado is, at heart, fuelled by local issues.
"The army, from the beginning… beat people up, took them to jail, tortured them. There's a lot of Islamophobia [in the majority Muslim province of Cabo Delgado]. They're discriminated against because they're northerners - people think they're dumb.
"The problem is that we have a youth bulge - and the young don't have jobs. If we solve… the abuse of force, corruption, and if we have a serious system of justice I'm sure we'll solve this very rapidly," said Professor Adam.
Government hiring foreign mercenaries
Mozambique's government initially sought to downplay the rebellion, dismissing the militants as criminals, and blocking journalists from accessing the region. But that is changing.
"We've seen a shift from the politics of denial. Most of society and politicians now accept we have an Islamist insurgency," said Mr Morier-Genoud.
Later, the government began to hire foreign security contractors - allegedly from Russia, the US and South Africa - to help the army crush the rebellion, but without any significant success.
There are concerns that the conflict, if mishandled, could spread into neighbouring Tanzania, and perhaps even to South Africa.
International gas companies - poised to invest billions in the off-shore gas fields discovered along the coast of Cabo Delgado - are now getting cold feet, partly because of the rising insecurity, but also because of falling gas prices.
Many observers and analysts believe that, fundamentally, the solution to the conflict lies in good governance, and a transparent attempt to address deep-seated economic and social grievances, including fair access to land, jobs, and a share of any future gas revenues.
"Multi-nationals want to know they can take their share, but they have to consider local people," said the Bishop of Pemba.
"And the government has to know that it is very necessary that Mozambique's natural resources must be used for the betterment of its people, not to cause corruption," he added.