Recent clashes and "indiscriminate attacks by security forces against civilian areas" have forced the population to flee their homes in eastern Kayah state near the Thai border, the international body's Myanmar office says.
An estimated 100,000 people have been displaced by new fighting between Myanmar's military and rebel groups in the east of the coup-stricken country, the United Nations said.
Myanmar has been in chaos and its economy paralysed since the generals ousted Aung San Suu Kyi's government in February, accusing it of fraud during 2020 elections.
Fighting has flared in several communities - especially in townships that have seen a high death toll at the hands of police - and some locals have formed "defence forces".
Recent clashes and "indiscriminate attacks by security forces against civilian areas" had forced an estimated 100,000 to flee their homes in eastern Kayah state near the Thai border, the UN's Myanmar office said on Tuesday.
Those in areas hit by fighting were in "urgent need" of food, water, shelter and health care, it said, adding that travel restrictions imposed by security forces were delaying the delivery of much-needed aid.
Recent violence in #Kayah State displaced an estimated 100,000 people. The @UN reiterates its earlier calls to take measures to protect civilians, to allow safe passage of humanitarian supplies & to facilitate the direct provision of relief assistance.— United Nations in Myanmar (@UNinMyanmar) June 8, 2021
'Artillery shells land in villages'
Locals in Kayah state have accused the military of using artillery shells that have landed in villages.
AFP images from the region have showed villagers manufacturing guns in makeshift factories as local defence groups go up against Myanmar's battle-hardened military.
More than 800 people have been killed across the country in a brutal military crackdown on dissent since February, according to a local monitoring group.
Myanmar's national economy and banking system have been paralysed since the army's power grab.
Livelihoods have been lost after strikes and factory closures, fuel prices have shot up and those lucky enough to have bank savings face day-long queues to withdraw their cash.
On Tuesday the Red Cross said it was urgently ramping up efforts to meet the humanitarian needs of 236,000 people in Myanmar, already reeling from the Covid-19 pandemic before the coup struck.
The announcement came after the charity's president Peter Maurer was granted a rare meeting with junta leader Min Aung Hlaing last week and called for increased humanitarian access to the country.