The UN Security Council will hold an emergency meeting at 1930 GMT Wednesday following reports of Russia's latest military incursion in Ukraine, diplomats said.
The meeting was requested by the United States after NATO said columns of Russian tanks, artillery and combat troops had entered eastern Ukraine in the past two days.
Russia, however, denied the claims by NATO.
The meeting will be an open debate with the 15 members of the council and also attended by Ukraine, the Australian chair of the council said.
Ukrainian Defense Minister Stepan Poltorak said Kiev was preparing for a new round of fighting after seeing "increased activity" by Russia and pro-Moscow rebels across the region.