NIAMEY (Reuters) - French and Nigerien troops were conducting operations on Thursday in a region of Niger where three U.S. Army Special Forces members were killed the day before, becoming the first American soldiers to die in West Africa in decades.

Three Nigerien soldiers were also killed and two U.S. soldiers wounded in the attack, which took place in a southwestern Niger region where insurgents are active, U.S. Africa Command spokeswoman Robyn Mack said.

France’s regional Barkhane force was asked to support a counterattack after the Niger and U.S. troops were ambushed, French army spokesman Colonel Patrick Steiger told a news conference in Paris.

“It’s not clear if the attackers knew the Americans were present,” said a Western security source. “Initial information suggests there was a trap that appeared designed to get them out of their vehicles and then they opened fire.”

Insurgents in the area include militants from al Qaeda in Islamic Maghreb and a local branch of Islamic State, Mack said. The Western security source said al Qaeda and a relatively new group called Islamic State in the Greater Sahara were the main suspects, although no one had yet claimed responsibility.

Two other Niger security sources said four military helicopters had been sent to the region and that reinforcements arrived on Thursday morning in the Tillaberi area, where the attack took place.

The Pentagon declined to discuss details, citing ongoing operations, but said the U.S. forces were part of a growing mission to bolster Niger’s armed forces’ ability to battle militants.

“We extend our deepest condolences and sympathies to the families of the fallen, and those injured in this attack,” Pentagon spokeswoman Dana White told a news briefing.

AMBUSH

Nigerien army Colonel Abou Tagué said the joint Nigerian-American patrol on the Mali-Niger border was ambushed by militants riding “in a dozen vehicles and about 20 motorcycles near the village of Tongo Tongo.”

He cited intense fighting, saying the joint Nigerian-American force “showed courage.” He said four Nigerien soldiers were killed and eight were wounded.

A statement by U.S. Africa Command on Thursday said one unspecified “partner nation member” had died but did not name the nation.

In a speech on Thursday, Niger President Mahamadou Issoufou condemned the attack. “Our country has just been the victim of a terrorist attack that claimed a large number of victims,” he said.

Islamist militants form part of a regional insurgency in the poor, sparsely populated deserts of West Africa’s Sahel. Jihadists have stepped up attacks on U.N. peacekeepers, Malian soldiers and civilian targets since being driven back in northern Mali by a French-led military intervention in 2013.

Malian militant groups have expanded their reach into neighboring countries, including Niger, where a series of attacks by armed groups led the government in March to declare a state of emergency in the southwest.

The European Union has pledged tens of millions of euros to a new regional force of five Sahelian countries - Niger, Mali, Chad, Burkina Faso and Mauritania - in a bid to contain Islamist militant groups. The United States also views the region as a growing priority.

Rinaldo Depagne, West Africa project director at International Crisis Group, said the borderlands between Mali, Niger and Burkina Faso were “becoming a new permanent hotbed of violence”, threatened by increasingly organized militant groups.

“This shows the level of organization of these groups and also their confidence,” Depagne said.

Andrew Lebovich, a visiting fellow at the European Council on Foreign Relations, said Wednesday’s attack revealed how U.S. training of Nigerien forces “has accelerated and also verged into ongoing military operations”.

The United States has about 800 service members in Niger, where it operates surveillance drones out of a $100 million base in the central city of Agadez to support the country’s efforts to combat jihadists and protect its porous borders.

It has also sent troops to supply intelligence and other assistance to a multinational force battling the Nigerian Boko Haram militants near Niger’s border with Nigeria.