Ukrainian airborne troops have landed at an airport in the east of the country, at the start of an apparent crackdown on pro-Russian separatists occupying government buildings in 10 localities near the Russian border.
Tuesday's landing at Kramatorsk triggered conflicting accounts of events at the airfield, with the Kyiv government reporting its capture. However, witnesses, including Western journalists, say the Ukrainian force was quickly surrounded by local civilians and did not immediately move toward the city itself. Separately, officials in Kyiv said a similar operation had been launched in the nearby town of Slovyansk.
Earlier, Interim Ukraine President Oleksandr Turchynov said government forces had begun a "cautious" operation in Ukraine's Russian-speaking east, after a deadline for armed protesters to surrender passed Monday with no movement.
The White House on Tuesday praised Kyiv for saying it intends to use restraint and gradually move to liberate the occupied facilities.
In New York, United Nations Secretary-General Ban Ki-moon spoke by telephone with Russian President Vladimir Putin. Mr. Ban voiced alarm about what he called the "highly volatile" situation in eastern Ukraine and said he urged "everyone to work to de-escalate the situation."
For his part, Mr. Putin -- widely accused outside of Russia of backing the building takeovers -- called on the United Nations to condemn the Ukrainian crackdown.
Russian Foreign Minister Sergei Lavrov warned that Moscow will withdraw from an emergency international summit scheduled for Thursday in Geneva if Kyiv uses force in eastern Ukraine.
Lavrov was questioned about the situation Tuesday during a visit to China.
A report issued Tuesday by the U.N. human rights commissioner accuses Russia of manipulating the March 16 vote in Crimea that Moscow used to justify the annexation of the peninsula. The report cited propaganda, harassment and torture of anti-Russian activists.
It also said ethnic Russians in eastern Ukraine have falsely claimed to be under assault, in a move to justify Russian intervention ahead of the referendum. Additionally, it warned that more Russian propaganda could impact Ukraine's May 25 presidential election.
U.S. President Barack Obama warned his Russian counterpart Monday that Moscow will face further costs if its actions in Ukraine persist. He also urged the Russian leader to use his influence to persuade the pro-Moscow demonstrators to leave the occupied buildings in Ukraine's east.
The United States has already imposed three rounds of sanctions against Moscow for its annexation of Ukraine's Crimean peninsula last month, and U.S. officials say more sanctions will target close Putin allies as well as Russian entities.