(CNN) -- A state of emergency has been declared in Bolivia's Santa Cruz state, one of four states in the nation battling wildfires, the state-run news agency reported.
The fires have burned more than 3.7 milion acres (1.5 million hectares) in the past few weeks and are advancing "dangerously" in four of the nation's nine states, the government's ABI news service said Wednesday.
In addition to Santa Cruz, the fires are burning large segments of Pando, Beni and La Paz states. The flames are fanned by high wind gusts and arid conditions caused by drought, officials said.
More than 50 homes have been destroyed, the Ministry of Environment and Water said Thursday.
Dense smoke from the out-of-control fires has led to the closureof 28 of the nation's 39 airports, said CNN affiliate Unitel TV. Visibility in some areas was less than a mile, officials said.
The smoke, which makes it hard to breathe and stings the eyes, also has affected the health of some residents, officials said.
President Evo Morales said at a news conference Thursday that Bolivia does not have helicopters, airplanes or other special equipment to combat the blazes.
The fires have been started by farmers clearing land so they can plant their crops. Livestock herders also typically clear brush and trees from land by burning foliage.
The environment and water ministry asked the farmers and herders Thursday to stop the yearly practice.