WASHINGTON, United States -- More than one third of the Amazon rainforest may have been degraded by human activity and drought, researchers said Thursday, and action is needed to protect the critically important ecosystem.

In a study published in the journal Science, the researchers said the damage done to the forest which spans nine countries is significantly greater than previously known.

For the study, they examined the impact of fire, logging, drought and changes to habitat along the forest borders -- what they called edge effects.

Most previous research into the Amazon ecosystem has focused on the consequences of deforestation.

The study found that fire, timber extraction and edge effects have degraded at least 5.5 percent of all remaining Amazonian forests, or 364,748 square kilometers, between 2001 and 2018.

But when the effects of drought are factored in, the degraded area increases to 2.5 million square kms, or 38 percent of the remaining Amazonian forests.

"Extreme droughts have become increasingly frequent in the Amazon as land-use change and human-induced climate change progress, affecting tree mortality, fire incidence, and carbon emissions to the atmosphere," the researchers said.

"Forest fires intensify during drought years," they said, warning of the dangers of "much larger megafires" in the future.

The researchers from Brazil's Universidade Estadual de Campinas and other institutions used satellite images and other data from 2001 to 2018 to reach their conclusions.

In a separate study published in Science of the human impacts on the Amazon, researchers from the University of Louisiana Lafayette and elsewhere called for action.

"The Amazon is perched to transition rapidly from a largely natural to degraded and transformed landscape, under the combined pressures of regional deforestation and global climate change," they said.

"The changes are happening much too rapidly for Amazonian species, peoples, and ecosystems to respond adaptively," they said. "Policies to prevent the worst outcomes are known and must be enacted immediately.

"To fail the Amazon is to fail the biosphere, and we fail to act at our peril," they said.

Brazil's new president, leftist Luiz Inacio Lula da Silva, has pledged to end deforestation of the Amazon by 2030.