Kabore's state and whereabouts have been a key issue since he was overthrown by mutineering soldiers on Monday, with the UN leading calls for his release.

"President Kabore is physically well, but I cannot say anything about his state of mind," said a source in Kabore's People's Movement for Progress (MPP) party.

Kabore "is still in the hands of the army, not in a military camp, but in a presidential villa under house arrest", the source said.

"He has a doctor available... (and) access to his mobile phone, but under surveillance, obviously," the source added.

The West African bloc ECOWAS has lashed out at the "military coup" -- the third in one of its 15 nations in less than 18 months -- and said it will meet virtually for an extraordinary summit on the Burkina Faso crisis at 1000 GMT on Friday.

Kabore, 64, was elected in 2015 following a popular revolt that forced out strongman Blaise Compaore, who came to power in a putsch in 1987.

He was re-elected in 2020, but the following year faced a wave of anger over a jihadist insurgency that has ravaged the impoverished West African country.

On Sunday, mutinies broke out in several army barracks a day after police broke up banned protests, and on Monday the rebels moved against Kabore.

The former French colony is now in the hands of the Patriotic Movement for Preservation and Restoration (MPSR) -- the name of a junta led by Lieutenant-Colonel Paul-Henri Sandaogo Damiba, a regional commander in the jihadist-torn east.

It has announced the suspension of the constitution and dissolution of the government and parliament.

The junta also closed land and air borders. The air borders were reopened on Tuesday, along with an easing of restrictions on some products moving across land borders.

A military source said that Damiba on Wednesday would meet with senior army officers and top civil servants to discuss management of the country pending the appointment of a new government.

- Resignation letter -

RTB television published on social media a handwritten letter that it said was written by Kabore, purportedly saying he was stepping down "in the higher interests of the nation".

The source on Wednesday confirmed that the handwriting was genuine, "although I can't say about the conditions in which he wrote it".

ECOWAS on Tuesday said Kabore's resignation had been "obtained under threat, intimidation and pressure from soldiers after two days of mutiny".

The MPP source also gave details about some of the key events on Monday.

As the revolt widened, Kabore was smuggled out of his residence by his bodyguards in an unmarked car and taken to a safe location, the source said.

"It was later, as pressure from the mutineers rose, that his guards, who were mainly gendarmes, had to leave him in the hands (of the putschists) and join them," the source said.

"The gendarmerie had no other choice but to join (the putschists) because the whole army was in favour of stripping the president of office."

- New West African coup -

One of the world's poorest countries, Burkina Faso has enjoyed little stability since gaining independence from France in 1960.

The latest coup has caused jitters in West Africa, where in less than 18 months two other French allies -- Mali and Guinea -- have been overtaken by the military.

And to the east, Chad -- a key ally in the fight against the jihadist insurgency that began in the Sahel nearly a decade ago -- has been run by a junta since its veteran president, Idriss Deby Itno, died fighting insurgents last April.

Criticism of Burkina's latest coup has come from the European Union and the African Union, as well as the United Nations.

"Democratic societies are a value that must be preserved. Military coups are unacceptable," UN Secretary General Antonio Guterres said on Tuesday.

"The role of the military must be to defend their countries and their peoples, not to attack their governments and to fight for power."

Earlier this month, ECOWAS ramped up sanctions against Mali after the country's military said it would be unable to fulfil a pledge of staging elections by the end of February.