MOSCOW — Declaring that Russian troops had crossed into Ukraine, President Petro O. Poroshenko on Thursday canceled a planned visit to Turkey and convened a meeting of the national security council to focus on the “marked aggravation of the situation” in the southeast of his country.
The meeting of the national security council will focus on shaping a response, and Ukraine will also request a meeting of the United Nations Security Council.
Mr. Poroshenko made his comments as the leader of the main separatist group in southeastern Ukraine said that up to 4,000 Russians, including active-duty soldiers on leave, had been fighting against Ukrainian government forces, Russian television reported.
The confirmation that thousands of Russian soldiers have been involved in Ukraine is likely to increase tensions with the United States and the European Union, which have threatened harsher sanctions against Russia as a result of any military incursion.
“There are active soldiers fighting among us who preferred to spend their vacation not on the beach, but with us, among their brothers, who are fighting for their freedom,” Aleksandr Zakharchenko, a rebel commander and the prime minister of the Donetsk People’s Republic, said in an interview on Russian state-run television.