Four Indigenous children were found alive on Friday after spending more than a month lost in the Colombian Amazon rainforest following a small plane crash, President Gustavo Petro announced.

"A joy for the whole country! The 4 children who were lost 40 days ago in the Colombian jungle were found alive," Petro wrote on Twitter.

His post included a photograph of several adults, some dressed in military fatigues, attending to the children who were sitting on tarps among the dense forest.

The missing children are aged 13, nine, four and one.

"Yes, the children have been found, but I need a flight or a helicopter to go and get them urgently," the children's grandfather Fidencio Valencia told AFP.

Originally from the Uitoto Indigenous group, the children had been wandering alone in the jungle since May 1, when the Cessna 206 in which they were traveling crashed.

The bodies of the three adults who had been with them -- their mother, the pilot and a relative -- were all found at the crash site by the army. (AFP)