"We consider that the Norwegian procedure ... is cause for concern," the UNHCR's representative in the Nordic region, Pia Prytz Phiri, told reporters at the close of a three-day visit.
"It's a problem for us that Norway considers Russia a safe asylum country," she said.
Norway decided in November that migrants who arrived in the Scandinavian country after having stayed legally in Russia had to be swiftly sent back to Russia -- without having their asylum applications considered -- since Oslo considered Russia a safe country.
Some 5,500 migrants arrived in Norway via the "Arctic route" through Russia last year.
After sending some 200 people back, Norway stopped returning migrants to Russia at Moscow's request, as Russian authorities cited "security reasons".
But Norway has said it is negotiating a resumption with Moscow.
The Norwegian return policy risks depriving some legitimate asylum seekers of their right to protection, the UNHCR warned.
An independent body within the Norwegian immigration authority, Landinfo, said it had been informed by the UN agency that Russia had in January 2015 expelled two Syrians back to their war-torn country.
Norwegian Immigration and Integration Minister Sylvi Listhaug, speaking at the same press conference as Phiri Wednesday, assured that "we are of course going to listen to what the UN has to say".
She added however that "there are situations where the UN recommendations go beyond what our commitments are in respect to international conventions".
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The UNHCR also said it was "deeply concerned" by Norway's plans to tighten rules for family reunifications.
The right-wing coalition government, which includes an anti-immigration party, has proposed that a person must have studied or worked for at least four years before being allowed to bring over their family.
The proposal, which is still in the early stages, would require parliament's green light and the government does not hold a majority.