Tunisia's biggest labour union is staging a nationwide strike to protest against the government's refusal to raise the salaries of 670,000 public servants.

Rail, bus and air traffic stopped while schools, ports, hospitals, government offices and state media were affected on Thursday following the strike by the powerful Tunisia General Labour Union (UGTT).

Tunisia is under pressure from the International Monetary Fund (IMF) to freeze public sector wages as part of reforms to help reduce the country's budget deficit.

International lenders have threatened to stop financing the economy, which has been in crisis since the toppling of President Zine El Abidine Ben Ali in 2011.

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Prime Minister Youssef Chahed said the state will provide minimum services in vital sectors, including aviation, ports, buses and trains.

Tunisia's state-owned airline, Tunisair, said it expected major disruptions to its flight schedule and urged customers to change bookings, adding that at least 16 flights could be postponed.

Chahed said the strike will be "very expensive" but the government could not raise wages disproportionately to the state's ability to afford it.

Sami Tahri, deputy secretary-general of the UGTT, said the government is under the dictates of the IMF and has chosen the difficult solution of confrontation with public servants.

Government and union sources told Reuters news agency that the government had proposed spending about $400m on pay rises while the UGTT asked for about $850m.

Tunisia struck a deal with the IMF in December 2016 for a loan programme worth around $2.8bn to overhaul its ailing economy with steps to cut chronic deficits and trim bloated public services, but the progress has been slow.

Tunisia's economy has been in crisis since Ben Ali's toppling, throwing the country into turmoil, with unemployment and inflation shooting up.

The government aims to cut the public sector wage bill to 12.5 percent of gross domestic product in 2020 from the current 15.5 percent, one of the world's highest levels, according to the IMF.