West African bloc ECOWAS on Monday condemned what it called an attempted coup in Burkina Faso.
It said it held the military responsible for the physical well-being of President Roch Kabore, whose whereabouts are unknown.
"ECOWAS is following with great concern the evolution of the political and security situation in Burkina Faso, characterised since Sunday 23 January by an attempted coup d'etat," the organisation said in a statement.
Kabore's exact whereabouts or situation were unknown on Monday morning, with conflicting reports circulating among security and diplomatic sources.
The government had denied that the army had seized power.
A Twitter post on Monday from Kabore's account called on those who have taken up arms to lay them down.
"Our nation is going through difficult moments. We must at this precise moment safeguard our democratic norms. I invite those who have taken up arms to lay them down in the higher interest of the nation," said the post, which was signed RK.
Kabore has faced waves of street protests in recent months as frustration has mounted over the frequent killing of civilians and soldiers by militants, some of whom have links to IS and Al Qaeda.
A militant attack in November on a gendarmerie post in Inata, in the northern Soum region, killed 49 military police officers and four civilians. It later emerged the forces stationed there had run out of food and been forced to slaughter animals in the vicinity for two weeks.
Protesters came out to support the mutineers on Sunday and ransacked the headquarters of Kabore's political party.
The government declared a curfew from 2000 GMT to 0530 GMT until further notice and closed schools for two days.
The turmoil in Burkina Faso comes after successful military putsches over the past 18 months in Mali and Guinea, where the army removed President Alpha Conde last September.
The military also took over in Chad last year after President Idriss Deby died on the battlefield there.
Burkina Faso is one of the poorest countries in West Africa despite being a gold producer.
Buffeted by street protests, Kabore had pledged in November to end "dysfunction" in the army, saying an inquiry into the Inata attack would be followed by disciplinary measures and that he would launch an anti-corruption drive.
Some of the anger in Burkina Faso late last year was also directed against former colonial ruler France, which has deployed thousands of soldiers in West Africa's Sahel region to combat the militants.