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.
Still no sign of Russia pullback - NATO
NATO says there is still no sign that Russian forces are being withdrawn from Ukraine borders after President Vladimir Putin announced a return to their bases.
"We have not seen a change at the border," a NATO official said on Tuesday when asked if there had been any developments subsequent to Putin's announcement the previous day.
NATO head Anders Fogh Rasmussen said on Monday that after what he believed to be the Russian president's third such statement, "unfortunately, we have not seen any evidence at all that Russia has started withdrawal".
NATO has estimated there are some 40,000 Russian troops on the border, whose presence has raised fears they may intervene in troubled eastern Ukraine after Moscow's annexation of Crimea in March.
A pullback could ease spiralling tensions five days ahead of a key presidential poll on Sunday.
Ukrainian Foreign Minister Andriy Deshchytsia said earlier on Tuesday that his government could not yet confirm Russia's claim it had pulled its troops back from his country's border.
"We hope that the declarations by Russian politicians that the troops are to be withdrawn from Ukraine's borders don't just remain declarations," he said in Berlin.
Ukraine border guards said earlier that Russian forces had left an area within 10km (6mi) of the frontier following Putin's statement in which he also called on Kiev to halt military operations against pro-Kremlin rebels in the country's east.