The latest upheaval in the upper echelons of power comes as the far-right president faces mounting pressure over a deadly surge of Covid-19 in Brazil, where the average daily death toll has nearly quadrupled since the start of the year to more than 2,600 -- pushing many hospitals to the brink of collapse.

On Monday, Bolsonaro gave his embattled administration a thorough shake-up, replacing the foreign, justice and defense ministers as well as his chief of staff, attorney general and government secretary.

Last week, he also replaced former health minister Eduardo Pazuello, an army general with no medical experience, with cardiologist Marcelo Queiroga, his fourth health minister of the pandemic.

Bolsonaro, who comes up for re-election in October 2022, faces a firestorm of criticism -- including from key allies in Congress and the business sector -- over his handling of a pandemic that has now claimed nearly 314,000 lives in Brazil, the second-highest death toll worldwide after the United States.

The defense ministry did not give a reason for the departure of Army General Edson Pujol, Navy Admiral Ilques Barbosa and Air Force Lieutenant-Brigadier Antonio Carlos Bermudes.

"The decision was communicated in a meeting Tuesday with incoming defense minister (Walter Souza) Braga Netto and outgoing minister Fernando Azevedo," it said in a brief statement.

Some Brazilian media reported the trio had resigned in protest at Bolsonaro's surprise decision Monday to replace Azevedo.

"For the first time in history, the commanders of all three branches of the armed forces presented their joint resignation in disagreement with the president," said newspaper Folha de Sao Paulo.

- 'Possible political crisis' -

Former defense minister Azevedo himself resigned because "he was uncomfortable with the expectation that he would formally back President Bolsonaro's stance when he used the military for political ends," journalist Merval Pereira wrote in newspaper Globo.

Bolsonaro, a former army captain, frequently boasts of having the army's backing, and has packed his government with generals and officers.

He is openly nostalgic for the military dictatorship that ruled Brazil from 1964 to 1985, despite its rights violations, including the kidnapping and torture of dissidents and suspected opponents.

There was no official word on the reasons for Azevedo's exit, but he said in a parting statement he was proud to have "preserved the armed forces as an institution of the state."

Political analyst Oliver Stuenkel said the turmoil in Brasilia was part of pre-election jockeying that has seen Bolsonaro strike a new alliance with a coalition of centrist parties in Congress, seeking to bolster his shot at winning a second term next year.

The president looks to be facing a tough electoral battle in 2022, particularly after a Supreme Court justice earlier this month overturned former president Luiz Inacio Lula da Silva's corruption convictions, clearing the way for him to mount a potential run against Bolsonaro.

Polls place the popular but controversial leftist leader (2003-2010) neck and neck with Bolsonaro, at a time when the coalition that brought the president to power in January 2019 is badly frayed.

"The latest changes have a twin purpose: first, because (Bolsonaro) needs to free up cabinet posts for his new centrist allies, and second, because he is preparing for a possible political crisis and wants to surround himself with extremely loyal people, especially in the armed forces," Stuenkel said.