An Azerbaijani presidency spokesman told AFP that a "UN mission arrived in Karabakh on Sunday morning" - mainly to assess humanitarian needs.
It marks the first time in about 30 years that the international body has gained access to the region.
Armenian separatists, who had controlled the region for three decades, agreed to disarm, dissolve their government and reintegrate with Baku following a one-day Azerbaijani offensive last week.
The end of Karabakh's separatist bid dealt a heavy blow to a centuries-old dream by Armenians of reuniting what they say are their ancestral lands, divided among regional powers since the Middle Ages.
Nearly all of Karabakh's estimated 120,000 residents fled the territory over the following days, sparking a refugee crisis.
Armenia, a country of 2.8 million, faces a major challenge housing the sudden influx of refugees.
Authorities said 35,000 were now in temporary accommodation.
The International Federation of Red Cross and Red Crescent Societies on Friday announced an emergency appeal for 20 million Swiss Francs ($22 million) to help those fleeing.
Azerbaijan is now holding "re-integration" talks with separatist leaders while at the same time detained some senior figures from its former government and military command.
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