By Andrew Downie

SAO PAULO (Reuters) - The United Nations has called on Brazilian authorities to investigate the killing of a scrappy local journalist in the nation's interior, the third reporter to die in the country this year.

Joao Miranda do Carmo was shot seven times on Sunday night by unknown assailants in Santo Antonio do Descoberto, a small city 30 miles (48 km) west of the capital Brasilia.

Miranda wrote a blog that covered local issues and frequently blamed authorities for problems such as unpaved roads, the non-payment of employees and irregularities in municipal tenders.

Irina Bokova, director-general of the U.N. Educational, Scientific and Cultural Organisation, condemned the killing.

"It is important that the authorities investigate this killing and bring its perpetrators to justice," Bokova said. "Impunity for crimes against journalists threatens media workers' ability to do their work and the public's access to independent and diverse sources of information."

Miranda, 54, was also a member of the local Communist Party of Brasil who planned to stand in municipal elections this year, and officials believe he was killed because of his advocacy.

"He was very controversial in his city; he demanded answers from politicians, police, and local officials," said Claudio Curado, president of the Goias State Union of Professional Journalists. "We believe the crime has a political component. He questioned local politicians."

The local mayor could not be reached for comment.

Miranda reported at least two previous death threats, and his car was set on fire in 2014, local police officer Gilson Ferreira said.

According to Reporters Without Borders, Miranda was the third journalist to be killed for his work this year in the South American nation.

At least 38 journalists have been killed in Brazil since 1992, according to the Committee to Protect Journalists.

(Editing by Lisa Von Ahn)