Thousands of Muslim Tatars have fled Crimea since Russia annexed the province, although most have stayed in Ukraine.
At least 10,000 people have been driven from their homes since the start of the Ukraine crisis, with Crimean Tatars the hardest-hit, the UN refugee agency says.
Crimea's Muslim Tatars, generally seen as pro-Kiev, have fled the southern peninsula in their thousands since a separatist referendum that led Russia to annex the province.
"Displacement in Ukraine started before the March referendum in Crimea and has been rising gradually since," Adrian Edwards, spokesman for the UN high commissioner for refugees, said as he released the figure on Tuesday.
"Most of those displaced are ethnic Tatars, although local authorities have also reported a recent rise in registrations of ethnic Ukrainians, Russians and mixed families," he told reporters.
The true figure may exceed 10,000, Edwards said, because that number only includes people who have registered with local authorities.
Most have stayed in Ukraine rather than seeking refugee status abroad, he said.
Last week, a UN human rights probe condemned what it said was the harassment and persecution of the Crimean Tatars, in language that sparked an angry rebuke from Moscow.
"Among accounts we're hearing from displaced people is that they have left either because of direct threats or out of fear of insecurity or persecution," said Edwards.
Almost half the 10,000 internally displaced people have headed to central Ukraine, and around a quarter to the country's west.
In the wake of the Crimea takeover, Ukraine is being rocked by fighting pitting government forces against pro-Moscow separatists in the heavily Russified east of the ex-Soviet republic.
Edwards said that he did not have separate figures for people fleeing from the east.
The UN has set up three offices to help the displaced, in the capital Kiev, the southern city of Kherson, and Lviv in the west.