Niger's new military rulers on Wednesday accused France, the country's traditional ally, of releasing captured jihadists and breaching a ban on air space on the eve of a key summit on the Sahel's latest crisis.

Leaders of the West African bloc ECOWAS are to meet in the Nigerian capital Abuja to weigh their options, with diplomacy apparently edging out military intervention for now.

Two weeks after a coup that toppled elected president Mohamed Bazoum, the regime accused France of having "unilaterally freed captured terrorists" -- a term for jihadists conducting a bloody eight-year-old insurgency.

The jihadists then allegedly gathered to plan an attack on "military positions in the tri-border area", a hotspot region where the frontiers of Niger, Burkina Faso and Mali converge, according to the statement issued by the coup leaders, called the National Council for the Safeguard of the Homeland (CNSP).

"Events of an extreme gravity are unfolding in Niger as a result of the behaviour of the French forces and their accomplices," it declared.

It urged security forces to "raise their alert level across the country" and the public "to remain mobilised and vigilant".

The regime also accused France of having flown a "military plane" from neighbouring Chad into Niger air space on Wednesday, defying a ban imposed on Sunday.

The French government rejected the allegations.

"France firmly denies the new unfounded accusations by the putschists in Niger," said a joint statement by the French defence and foreign ministries.

- Key summit -

They also rejected the claim that France, which has around 1,500 soldiers stationed in Niger to aid anti-insurgency efforts, had released any jihadist prisoners.

It added that the flight had been approved and coordinated with Nigerien armed forces, "an authorisation confirmed in writing".

Thursday's summit of the Economic Community of West African States (ECOWAS) takes place under the chairmanship of Nigeria, the regional superpower and advocate of a hard line against the coup.

Struggling to stem a cascade of coups among its members, the bloc gave Niger's military rulers until last Sunday to reinstate Bazoum or face potential use of force.

But the coup leaders remained defiant, and the deadline passed without action.

On Tuesday, a bid to send a joint team of ECOWAS, UN and African Union (AU) representatives to the capital Niamey was rejected by the coup leaders.

But in a twist a day later, a former emir of the Nigerian city of Kano revealed that he had met with the coup leaders to help mediate the crisis.

Sanusi Lamido Sanusi told Nigerian state television he had spoken to coup leader General Abdourahamane Tiani and would deliver a "message" to Tinubu, though he was not an official government emissary.

"We came hoping that our arrival will pave the way for real discussions between the leaders of Niger and those of Nigeria," said Sanusi, who is known to be a close friend of Tinubu.

ECOWAS and Nigeria have said they will pursue all options to resolve the crisis, and the United States emphasised hopes for a "peaceful" outcome even though it too had been rebuffed.

- Fears over Bazoum -

The leader of the United Nations on Thursday added to a chorus of concern about the welfare of 63-year-old Bazoum, who was detained on July 26 by members of his presidential guard.

Secretary-General Antonio Guterres denounced "the deplorable living conditions that President Bazoum and his family are reported to be living under", according to a UN statement.

CNN reported Wednesday that Bazoum was being kept in isolation and forced to eat dry rice and pasta.

In a series of text messages he sent to a friend, the ousted president said he had been "deprived of all human contact since Friday", with no one supplying him food or medicine, the network reported.

US Secretary of State Antony Blinken said he was "greatly worried" about Bazoum's safety, after speaking to him by phone.

It is the fifth coup in Niger since independence from France in 1960 -- and the fourth in the ranks of the 15-nation ECOWAS since 2020.

Bazoum's election in 2021 had helped Niger cement close ties with France and the United States, which have major bases and troop deployments in the country.

- Sahel crisis -

Countries in the fragile Sahel are battling a jihadist insurgency that erupted in northern Mali in 2012, spread to Niger and Burkina Faso in 2015, and is now causing jitters in states on the Gulf of Guinea.

The bloody campaign has been devastating for those three countries, which have turbulent histories and are among the poorest nations in the world.

Mali and Burkina Faso have also seen coups in recent years and fallen out with France, the region's former colonial power.

France last year withdrew its forces from those countries and refocussed its anti-jihadist strategy on Niger.