KIEV, Ukraine — Ukrainian officials said Wednesday that two military jets have been shot down in the eastern part of the country, and the missiles that downed them may have been launched from Russia.

There was no immediate response from Russian officials. The state-run Itar-Tass news agency reported that the planes were shot down by a separatist militia.

The pilots of the Su-25 fighter aircraft ejected from the planes, the Ukrainian government said, but their whereabouts are unknown. The incidents occurred near the Russian border, in a part of the country where the government is waging a battle against pro-Russia separatists.

However, at an afternoon security briefing, a Ukraine defense spokesman said the military believes that the missiles that brought down the planes came not from separatists in Ukraine but from the Russian side of the border.

“They were downed not by terrorists,” said Andriy Lysenko, spokesman for the National Security and Defense Council, using the government’s term for the separatists. “?According to our preliminary information it was done from across the border.”

Lysenko cited the fact that the planes were flying? at an altitude of more than 15,000 feet as part of the evidence that the missiles that brought the aircraft down couldn’t have come from separatists.

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Asked if this contradicted the government’s belief that separatists had access to just such a capability in bringing down Malaysia Airlines Flight 17, which was flying at twice that altitude, Lysenko said, “This is just based on preliminary data” and did not elaborate further, saying only that “an investigation is being arranged.”

He said the military still did not know the whereabouts of the pilots.

The news comes less than a week after the downing of Malaysia Airlines Flight 17 by a missile suspected to have been fired by separatists.

The pro-Russia paramilitary units have been firing at jets above eastern Ukraine airspace in recent months, bringing down two military transport planes in the days before the Malaysia Airlines disaster. But, if confirmed, the attack Wednesday would be the first strike in the area since the downing of the commercial jet.

Despite the general proximity to the Malaysia Airlines crash site, one of the reported shootdowns Wednesday, over Lysychansk, is about 100 miles north of Grabovo, where much of the wreckage from the Malaysian plane ended up last week.

The separatists have denied any involvement in the Malaysia Airlines strike, and Russia has denied supplying them with the weapons systems required for such an attack. However, the U.S. and other nations believe that Russia has been providing the separatists with surface-to-air missiles, including the kind suspected in the Malaysia Airlines downing, and have demanded that Moscow halt support of the breakaway group.