OUAGADOUGOU - Armed men attacked the French embassy in Burkina Faso and the country's military headquarters on Friday before being repelled in a battle that left dozens of dead or injured.

Local security sources put a still-incomplete death toll at about 15 while other sources reached by AFP from Paris sketched an even bloodier outcome, with at least 28 people killed in the second assault alone.

The coordinated operation underscored the fragility of the Sahel nation, one of a string of African states struggling with a bloody militant insurgency. Heavy gunfire broke out mid-morning in the centre of the Burkinabe capital Ouagadougou.

Witnesses said five armed men got out of a car and opened fire on passersby before heading towards the French embassy. The car was later seen ablaze.

At the same time, an explosion occurred near the headquarters of the Burkinabe armed forces and the French cultural centre, which are located about a kilometre (half a mile) from the site of the first attack, other witnesses said.

Burkinabe security sources said around 15 people were killed, while the army's medical chief, Colonel Amado Kafando, said 75 others had been injured. It was unclear whether these figures applied to one or to both attacks, and whether they included the assailants.

Earlier, the government had announced that six assailants had been killed - four in the embassy attack and two in the military attack - while state TV said seven members of the security forces had also died. Three security sources - two in France and one in West Africa, told AFP that at least 28 people were killed in the attack on the military HQ alone.

French government sources said there had been no French casualties and described the situation in Ouagadougou as "under control".

Information Minister Remis Fulgance Dandjinou said the attack "has strong overtones of terrorism." The Paris public prosecutor said it had opened a formal investigation into "attempted murder in relation to a terrorist enterprise", an expected procedural step after attacks targeting French citizens or interests. Burkina Faso is one of a group of fragile countries on the southern rim of the Sahara that are battling militant groups.

The insurgency has caused thousands of deaths, prompted tens of thousands to flee their homes and dealt crippling blows to economies that are already among the poorest in the world.

On August 13 last year, two assailants opened fire on a restaurant on Ouagadougou's main avenue, killing 19 people and wounded 21. The attack remains unclaimed.

On January 15 2016, 30 people, including six Canadians and five Europeans, were killed in a militant attack on a hotel and restaurant in the city centre. Responsibility was claimed by a group called Al-Qaeda in the Islamic Maghreb (AQIM). France, the former colonial power in the Sahel region, has deployed 4,000 troops and is supporting a five-country joint force gathering Burkina Faso, Chad, Mali, Mauritania and Niger. On February 21, two members of the French counter-terrorism force were killed by a landmine near Mali's border with Niger and Burkina Faso. Twelve French troops have died since the campaign, called Operation Barkhane, was launched in August 2014. The United Nations also has a 12,000-strong peacekeeping force in Mali called MINUSMA, which has taken heavy casualties. Four UN peacekeepers were killed by a mine blast on Wednesday in the centre of the country.

In a separate development on Friday, the specialist US website SITE, which monitors militant activity, said kidnappers had released a video of a 75-year-old French hostage, Sophie Petronin, who had been abducted in northern Mali in late 2016.

Petronin, who had been running an association helping Malian orphans, appears in poor health in the brief video.

Her kidnapping, hitherto unclaimed, was carried out by the "Support Group for Islam and Muslims". In the background, Macron's voice is heard on a loop, saying "I will protect you".