Though there is a growing body of evidence that Russia has provided tanks, weapons and other support to the separatist rebels, and several leaders of the insurrection have identified themselves as Russian citizens, there has been no proof of active engagement in Ukraine by the regular Russian military. The Ukrainian government, however, has complained on several occasions of violations of its airspace by Russian aircraft.

Mr. Lysenko said the pilot of the Su-25 fighter had ejected from the aircraft and parachuted safely to the ground, and had been removed to safety by Ukrainian ground forces. “The pilot had no injuries,” he said.

Although President Vladimir V. Putin of Russia has called repeatedly in recent weeks for a renewal of peace negotiations that would directly involve separatist leaders, the United States has said Russia has stepped up its military support for the rebels. On Wednesday, President Obama announced additional economic sanctions to penalize Russia for its role in the Ukraine crisis.

“Even in the face of all this diplomacy that’s been going on and very high-level efforts by us and by key members of the European Union, notably France, Germany, the U.K., in fact, over the past month, the flow of heavy weapons and support for separatists from Russia has actually increased,” a senior Obama administration official said on Wednesday, during a briefing for reporters about the new sanctions.

“You will have seen on social media over the last week convoys of Russian tanks, armored personnel carriers, infantry combat vehicles, Grad rocket launchers, Howitzers, self-propelled mortars flowing into Ukraine,” said the official, who spoke on the condition of anonymity under the ground rules that the White House has imposed for such briefings.