A United Nations mission arrived Sunday for the first time in about 30 years in the mountainous territory after Azerbaijani forces ousted ethnic Armenian separatists and almost the entire population of the territory fled to Armenia.

The team departed after one day in the enclave, with UN spokesman Stephane Dujarric telling reporters Monday that the mission to assess humanitarian needs "did not come across any reports... of violence against civilians" following the latest ceasefire in the wake of last month's lightning Azerbaijani takeover.

UN representatives visited the eerily silent Stepanakert, the region's capital, and "saw no damage to civilian public infrastructure," Dujarric said in New York, adding: "However, no shops seemed to be open."

In Washington, State Department spokesman Matthew Miller said a longer-term force could monitor the situation for the sake of residents who wish to return.

"We are reiterating our call for an independent international monitoring mission that would provide transparency and reassurance to the population of Nagorno-Karabakh that the rights and (security) of ethnic Armenians would be protected, particularly for any of those that wish to return," Miller told reporters.

"Azerbaijan has made those assurances. We think there ought to be an international monitoring mission there to observe and guarantee them," he said.

He declined to weigh in on whether there had been "ethnic cleansing," as claimed by Armenians, but said the United States took the allegations "seriously."

"We won't shy from taking appropriate actions to respond to allegations of atrocities and promote accountability for those responsible," Miller said.

"But as always, the determination regarding genocide or ethnic cleansing is based on a deliberate, evidence-based process."