(AFP) - The United Nations expressed concern Tuesday over the repatriation of four Turkish refugees from Kenya who rights groups say were abducted and forcibly returned in violation of international law.

The four -- Mustafa Genc, Huseyin Yesilsu, Ozturk Uzun and Alparslan Tasci -- were sent back to Turkey on Friday, according to the Kenyan foreign ministry.

It followed media reports that they had been kidnapped on the street in the Kenyan capital Nairobi along with three others who were later released.

UNHCR, the UN refugee agency, said in a statement to AFP that it was "deeply concerned by the refoulement of four refugees from Kenya".

It called on Kenya`s government "to abide by their international legal obligations and in particular, to respect the principle of non-refoulement, which protects asylum-seekers and refugees from any measure that could lead to their removal to a place where their life or freedom would be threatened".

Kenya`s foreign ministry said it had received assurances from Turkey that the four will be "treated with dignity".

"This incident constitutes a breach of both Kenya and international refugee law," Amnesty International said on Saturday before the repatriation had been confirmed.

"Their abductions underscore the growing concerns about the safety of all refugees and asylum seekers in Kenya."

An alliance of Kenyan rights groups, the Police Reforms Working Group, said it was "shocked" by the government`s action, saying it had "placed four human beings at grave risk as well as Kenya`s standing as a sanctuary nation".

It said the action "undermines Kenya`s credibility" as a new member of the UN Human Rights Council.

Kenya is hosting more than 780,000 refugees, the foreign ministry said.