Brazil's President Jair Bolsonaro is suffering from an "intestinal obstruction," the government said on Wednesday after the far-right leader was admitted to hospital due to persistent hiccups.

The 66-year-old far-right leader had been complaining publicly since last week about suffering from hiccups following a July 3 surgery on a dental implant.

Bolsonaro was first taken to a military hospital in the capital Brasilia but was then flown in a Brazilian Air Force plane to Sao Paulo where he was admitted to the Vila Nova Star private hospital.

There he underwent clinical, laboratory and imaging tests and was being kept in the hospital for the time being, doctors said. 

"He is in good spirits and feels well," the presidency had said earlier in the day.

Bolsonaro was examined in Brasilia by Antonio Macedo, who has operated on the president several times since he was stabbed in the abdomen in 2018 while on the campaign trail.

The hospital said he would "initially" be given a "conservative clinical treatment."

If it is confirmed he needs an operation, it would be the president's seventh surgery since he was stabbed by a former member of the Socialist and Freedom Party (PSOL), a group that broke away from the Workers' Party (PT) of former president Luiz Inacio Lula da Silva, who is eyeing a run against Bolsonaro next year.

"Yet another challenge, a consequence of the assassination attempt carried out by a former member of PSOL, the left-wing arm of the PT, to prevent the victory of millions of Brazilians that wanted change in Brazil," Bolsonaro wrote on Twitter.

"A cruel attack not just against me but against democracy."

Speaking to a local radio station last week, Bolsonaro had said: "This happened to me before, maybe due to the medicine I'm taking, I have hiccups 24 hours a day."

On Tuesday night, a tired-looking Bolsonaro had complained to supporters outside his official residence in Brasilia about the problem.

"People, my voice has gone. If I start talking a lot, the hiccups return... they already have," he said.

Bolsonaro also contracted Covid-19 last year, although his symptoms were mild and he did not need hospital treatment.

His admission to hospital came after the Senate launched an investigation over his handling of the pandemic, the seriousness of which he repeatedly downplayed and which has cost almost 540,000 lives in Brazil. The investigation was extended by 90 days on Wednesday.

Bolsonaro is also being investigated by the public prosecutor for allegedly failing to act on an embezzlement tip-off regarding coronavirus vaccine purchases. 

The opposition is pushing for the chamber of deputies president to open impeachment proceedings against Bolsonaro.