Six volunteer firefighters use machetes to cut a path through the vines and underbrush of the Chiquitano forest in Bolivia's eastern lowlands. They're approaching the leading edge of a fire that's been burning for hours.

They attempt to smother it with shovelfuls of dirt and water they carry on their backs in tanks normally used to fumigate crops. But the smoke is getting thicker, the heat stronger and swirling winds push the flames forward. Realizing they are overmatched, José Zapata, the only trained firefighter among the group, orders his men to pull out.

Updated at 9:53 a.m. ET

President Trump has named Robert C. O'Brien, who has been his special envoy for hostage affairs, to be his new national security adviser.

Trump made the announcement in a Wednesday morning tweet.

"I am pleased to announce that I will name Robert C. O'Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job!" Trump said.