Six former journalists and staff of an opposition newspaper in Turkey have returned to prison to serve their sentences after an appeals court confirmed their conviction on terror charges.

Cartoonist Musa Kart and five other employees of the Cumhuriyet newspaper entered prison in northwest Turkey on Thursday after an appeals court in February upheld their conviction on charges of aiding terror organizations.

The six were sentenced to less than five-year prison terms. Other Cumhuriyet employees, who were sentenced to more than five years in prison, can still appeal the verdicts at Turkey's Supreme Court.

The paper is one of few newspapers critical of President Recep Tayyip Erdogan and their case has increased concerns over press freedom in Turkey.

The Turkish Journalists Syndicate says 133 journalists and media employees are currently in jail.