The Blackhawk helicopter was carrying 17 military personnel when it went down. The armed forces high command did not reveal whether it was shot down or an accident.

Authorities found the helicopter in a stretch of the river Inirida in Guaviare state, an area where dissident former Revolutionary Armed Forces of Colombia (FARC) guerrillas are active.

FARC rebels laid down their arms in a historic 2016 peace deal that ended a half century of conflict and turned the guerrillas into a communist political party.

But more than 2,000 dissidents, according to authorities, refused to join the 13,000 rebels that signed the peace deal and continue to resist the government, financing themselves though drug-trafficking and illegal mining.

The army said the helicopter was taking part in an operation against dissidents in an area where there are drug plantations.

The army's air assault division said in a statement it was "on site beginning the corresponding investigations to determine the circumstances regarding the time, manner and place that the events occurred."

The armed forces' top commander, General Luis Fernando Navarro traveled to a military base nearby to direct the "manoeuvres aimed at recovering the missing."

Defense Minister Carlos Holmes Trujillo referred to the incident as a "plane crash" but that version has not be corroborated by the military high command.

Trujillo said on Twitter that the six injured service personnel "have already been rescued and will be transferred to Bogota for their medical attention."

The banks of the Inirida river are in a strategic zone for the drug-trafficking trade where one of the most powerful criminal groups to emerge from FARC dissidents, led by Gentil Duarte, operates.

Duarte originally took part in peace talks but finally opted to continue the guerrilla struggle and is trying to unite other groups that broke away from the FARC peace movement.

Peace has not been easy with the former rebels complaining that 219 ex-fighters have been murdered since the the peace deal was signed.

Despite FARC laying down its arms, a multi-faceted conflict continues to rage.

Alongside FARC dissidents, another left-wing guerrilla group, the National Liberation Army (ELN), right-wing paramilitaries and drug-traffickers continue to battle each other and the Colombian army.

The more than half century conflict left nine million people dead, missing or displaced.