Four Indigenous children missing in the Colombian Amazon for almost three weeks have the intelligence and know-how to survive, their grandfather Fidencio Valencia has said.

The eldest of the four siblings, a girl named Lesly, "is very intelligent, very active, strong," said Valencia told the AFP news agency on Friday.

The others are Soleiny [nine], Tien Noriel [four] and 11-month-old Cristin.

Authorities have deployed 150 soldiers and sniffer dogs to the jungle region in southern Colombia to search for the four children, who they believe survived an airplane crash on May 1 that killed three adults.

The plane — a Cessna 206 — was carrying seven people on a route between Araracuara, in Amazonas province, and San Jose del Guaviare, a city in Guaviare province, when it issued a mayday alert due to engine failure in the early hours of May 1.

Indigenous people have joined the search operation while three helicopters have circled overhead looking for signs of the children.

Several clues to their continued survival have been found, including a baby's bottle, scissors, hair ties, shoes, clothing and even a footprint in the mud.

Soleiny and Tien Noriel "are very skillful at going" through the jungle, said Valencia, 47.

Above all, the siblings "love each other a lot."

But there are many dangers in the jungle, such as jaguars, pumas and venomous snakes, not to mention the difficulties of moving through thick vegetation where storms are frequent.

Being Indigenous is an advantage, said Valencia, a member of the Witoto community.

"You know some fruits, some plants, some leaves, something that can be useful. There is knowledge," he added.

Country on tenterhooks

The entire country has been left on tenterhooks waiting for news of the children.

President Gustavo Petro lifted and then crushed the nation's hopes when he announced on Wednesday that all four had been found alive and well, only to retract that a day later.

Valencia urged the rescuers to "not give up because the children are alive."

Faithful to his Indigenous beliefs, Valencia believes dark forces are at work hindering the rescuers' efforts.

He pointed to night and rain falling on Thursday right after the footprint was discovered.

"This means there is mystery in the jungle, suddenly there are dark forces holding the children," said Valencia.

He called for "more help on the ground, more dogs, more soldiers, more energy and more prayers both from the state and Indigenous people."