The United Nations Children's Fund (Unicef) said it was extremely "concerned and disappointed" with the Sri Lankan government's decision to expel the organisation's chief of communications in Sri Lanka, James Elder.
The Sri Lankan government cancelled Elder's visa without giving reasons, but it was widely believed that he was being expelled for recent remarks about the plight of refugees living in government-controlled camps in the north of the country.
"Through Mr Elder, Unicef has consistently spoken out against the suffering of children on both sides of the intense hostilities earlier this year and called for their protection. Unicef unequivocally rejects any allegation of bias," Unicef chief executive Ann Veneman said in a statement released in New York.
"Unicef has always upheld the principle of impartial advocacy and communication on behalf of children as a fundamental part of its global mandate," she said.
"Unicef will continue to uphold its mandate in Sri Lanka, and elsewhere, to advocate and speak out on behalf of vulnerable children and women," the statement added.
The Sri Lankan government has so far made no official statement on the expulsion.
Elder, who holds an Australian passport, had been working for Unicef in Sri Lanka since July 2008 and had a residency visa valid until 2010.
After his visa was cancelled he was given two weeks to leave.
Elder was quoted in the media saying the about 270,000 displaced people were suffering hardship due to heavy flooding in the camps after heavy rain in some areas of Vavuniya district.
The population was displaced due to fierce fighting between government troops and rebels of the Liberation Tigers of Tamil Eelam. The fighting ended May 18 when the rebel leadership was defeated.
The government has been under pressure to resettle the refugees, but it says mine clearing and infrastructure rebuilding must be done first.
The visas of at least 10 foreign workers of non-governmental organisations have been cancelled or not extended over allegations of bias against the Tamil rebels.