The UN agency said the proposals "could fuel fear, xenophobia and similar restrictions that would reduce - rather than expand - the asylum space globally and put refugees in need at life-threatening risks".
Danish lawmakers will discuss the merits of the proposed legislation in three separate sessions before a final vote. Current law can keep refugee families apart for up to one year.
"It's crazy talk," says Jonas Keiding Lindholm of Save the Children in Denmark. "Denmark has an obligation to process family reunification applications in a positive, humane and speedy fashion. But the government wants to do the very opposite."
Denmark's Liberal government has been swift to tighten immigration policies since the June elections. Rasmussen can't afford to back down for fear of losing the support of the anti-immigration Danish People's Party, which enjoys the backing of about one-fifth of the electorate, making it the biggest party in the ruling bloc.
But the balancing act of Danish multi-party politics is making life hard for Rasmussen. Another support party, the Liberal Alliance, has threatened to withdraw backing if tax cuts aren't delivered.
Rasmussen's foreign policy has soured relations with Sweden, which has accused the Danes of shirking their responsibility to those fleeing the crisis in the Middle East.
Denmark's decision to impose controls on the German border drew concern from Berlin over the future of the passport-free Schengen zone.
Denmark's treatment of asylum seekers who make it through the country's tough checks has also drawn criticism. The Danish Red Cross has slammed the Government for putting refugees in tents in sub-zero temperatures, when it says they could comfortably be lodged in more permanent quarters. A proposal to confiscate refugees' valuables to help cover their costs met international outrage, prompting the Government to soften its plan.