BRASILIA (Reuters) - Brazilian customs workers will start an indefinite strike on Thursday over wage increases, their union said on Tuesday, just three weeks ahead of the Olympics when thousands of tourists are expected to flock the South American country.

The Sindifisco union of federal tax auditors, who are in charge of customs and other tax monitoring duties, voted on Friday for the strike to pressure the government to honor its promise to raise their wages by 5.5 percent starting in August, union President Claudio Damasceno said in an interview.

A crippling political and economic crisis as well as the spread of the mosquito-borne Zika virus have raised questions about Brazil's ability to successfully host the world's biggest sporting event for the first time ever in South America.

Local authorities expect more than 500,000 foreign tourists to land in the coastal city of Rio de Janeiro for the Olympics, which start on Aug. 5.

"If the strike continues by then it will certainly disrupt the Olympics," said Damasceno, whose union represents 10,400 tax auditors across the country. "We know this is an important international event and hope that the government can resolve this issue before it starts."

Tax auditors plan to down tools every Tuesday and Thursday until the government issues a decree to raise their wages, Damasceno said.

The planning ministry said in a statement that it was reviewing wage agreements inked by suspended President Dilma Rousseff earlier this month.

Rousseff has been suspended while she faces an impeachment trial for allegedly breaking fiscal rules. She has been replaced by her vice president, Michel Temer.

The strike would cost the cash-strapped government about 1.5 billion reais ($455 million) in tax revenue every month, Damasceno estimated.

(Reporting by Alonso Soto; Editing by Richard Chang)