SEOUL (Reuters) - Malta has denied visa extensions for North Korean workers, effectively expelling them, due to a diplomatic campaign by South Korea and human rights groups, South Korea's Yonhap news agency reported on Thursday.

South Korea has been pressing the few countries that have ties with North Korea to push for changes in the isolated state, which has been criticized for its human rights record and for channeling wages earned by its workers abroad back to the North.

Malta has denied visa extensions for about 20 North Korean workers who had been employed at a construction firm and a clothing maker there, Yonhap reported, quoting unidentified sources in the Malta government and the companies.

All of the workers had since left Malta and returned to North Korea, Yonhap reported.

A diplomatic source in the capital, Valletta, said Malta had taken the move after a push by South Korea and human rights groups that raised concerns about the conditions faced by the North Korean workers, according to Yonhap.

Officials at Malta's consulate in Seoul could not be reached for comment.

North Korea has come under growing diplomatic pressure since its January nuclear test and a long-range rocket launch in February, which led to a new U.N. Security Council resolution in March tightening sanctions against Pyongyang.

South Korea has been making diplomatic efforts to engage North Korea's old allies to press for change in the isolated state.

Earlier this year, Namibia halted ties with two North Korean state-run companies that had built a munitions factory and were involved in projects for its military to comply with U.N. sanctions on Pyongyang, according to Namibian media and the South Korean government.

Poland has not issued new visas for North Koreans this year in reaction to the North's nuclear test and rocket launch at the beginning of the year amid concerns that Pyongyang may be subjecting its workers to conditions that violated their rights.

(Reporting by Jack Kim; Editing by Paul Tait)