More than the 4,000 estimated by Aleksander Zakharchenko, the pro-Russian rebel leader in eastern Ukraine.
Daniel Baer is the U.S. ambassador to the OSCE.
More from our news desk on Poroshenko's statement:
Ukrainian President Petro Poroshenko says a Russian military invasion of his country has occurred following reports that the southeastern border town of Novoazovsk has fallen to Russian troops.
Poroshenko canceled a planned trip to Turkey on August 28 and said he is convening an emergency meeting of the National Security and Defense Council to discuss the "sharp deterioration" of the situation in the Donetsk region.
The council said on Twitter on August 28 that Russian troops had seized control of Novoazovsk and several settlements in the Novoazovsk, Starobeshevskiy, and Amvrosiyivskiy districts.
The council said Ukrainian troops near Novoazovsk came under attack by Grad missile systems fired from Russian territory before being attacked by two convoys of Russian armor in Ukraine.
Western officials have accused the Russian military of directly intervening in eastern Ukraine.
Moscow has long denied arming the rebels or sending troops into Ukraine.
BREAKING: Ukrainian President Petro Poroshenko says a Russian military invasion has occurred and that he is convening an emergency meeting of the National Security and Defense Council to discuss the crisis. He has canceled a trip to Turkey.
BREAKING: Ukraine's Security and Defense Council says on Twitter that Russian troops have seized control of the border town of Novoazovsk. The town's mayor earlier said that pro-Russian fighters had entered the town with support from dozens of armored vehicles.
More on the case of Nadiya Savchenko:
A Russian court has refused to free a Ukrainian Air Force officer on bail and ordered her to be sent to Moscow for a psychiatric examination.
Senior Lieutenant Nadiya Savchenko was captured in July and is awaiting trial in Russia.
She is charged with complicity in the killing of two Russian journalists who died while covering the conflict between pro-Russian separatists and government forces in eastern Ukraine.
At a hearing on August 27, a court in the city of Voronezh refused to release her on bail and extended her pretrial detention through October 30.
It ruled that she will be sent to Moscow for a psychiatric examination.
Savchenko's lawyer, Mark Feigin, says his client has lodged a protest against the psychiatric examination.
Savchenko told the court that she is not guilty and that Russia has no right to try her.