Ukraine’s top defense official said Monday that Russia may have shot down a Ukrainian military transport plane, an action that would represent a dramatic new step in the months-long conflict that has engulfed eastern Ukraine.

Pro-Russian separatists immediately asserted responsibility for the downing of the An-26 plane over Luhansk, but Ukrainian Defense Minister Valery Heletey said the aircraft had been flying too high for their shoulder-mounted rockets to reach it.

“The plane was downed by another, more powerful missile weapon that probably was used from the territory of the Russian Federation,” Heletey told President Petro Poroshenko, according to a statement on the president’s Web site. Russia did not comment on the accusation.

Ukrainian authorities said that the plane was flying at about 21,000 feet and that they believed it had been hit either by a powerful Pantsir surface-to-air missile or by an air-to-air homing missile fired by a fighter jet from Russia’s Millerovo air base, near the Ukrainian border, a spokesman for Ukraine’s National Security and Defense Council, Andriy Lysenko, told reporters in Kiev.

The crew managed to parachute out before the plane crashed, and all survived, Lysenko said. Four crew members were taken captive by rebels, Russia’s Interfax news service reported separatist leaders as saying.

A Ukrainian An-26 military transport plane was shot down Monday by a missile over the village of Davydo-Mykilske, east of Luhansk, near the Russian border. (Dominique Faget/AFP/Getty Images)

Ukraine’s claim came as a NATO official said Russia has massed 10,000 to 12,000 troops near the Ukrainian border, after their number dropped to fewer than 1,000 in mid-June. “Since that time, the Russians have been building up their forces again,” the official said, speaking under alliance ground rules of anonymity.

Fears of an open Russian intervention in Ukraine have spiked in recent days, particularly after Moscow on Sunday accused Ukraine’s military of killing a Russian citizen on Russian soil after a shell apparently fired from Ukraine landed over the border. Ukrainian officials denied shooting into Russian territory, but Russia’s Foreign Ministry warned of “irreversible consequences,” and a senior legislator called for precision strikes against Ukrainian military installations.

Officials in Kiev have accused Russia in recent days of allowing a significant flow of heavy military equipment over its border into Ukraine. Russia has denied the charge.

Meanwhile, in an attempt to restart peace talks with the separatists, a Ukrainian government negotiating team will hold a videoconference Tuesday with rebel representatives, German Foreign Minister Frank-Walter Steinmeier said Monday after a conference call with top officials from Ukraine, Russia and France.

Also Monday, Russian Foreign Minister Sergei Lavrov invited observers from the Organization for Security and Cooperation in Europe to come to two Russian border posts to monitor the situation, a step that Russia previously said would be permissible only if Ukraine agreed to a cease-fire. It was not clear whether the addition of observers would do much to restrict traffic on the long border between the two countries.

In a sign that Ukraine may be trying to dial down tensions with Russia over Sunday’s border incident, Poroshenko announced Monday that he would be “narrowing” military operations in eastern Ukraine to bolster border security and to “ensure the protection of civilians.”

“If you gain the power, it is important not only to fly the flag but also to protect the flag, state and people,” Poroshenko said, promising that the military would turn its attention to rehabilitating infrastructure and civil services in areas “liberated” from pro-Russian separatists.

Smoke rises from the wreckage of the military transport plane. (Dominique Faget/AFP/Getty Images)

Since mid-April, the Ukrainian military has been trying to push back the separatists. On July 5, it had its first major successes, reclaiming control of the cities of Slovyansk and Kramatorsk.

But the fighting between the military and separatist militias continues to rage elsewhere in the region, particularly in Luhansk, a city of 430,000 where Ukrainian forces made significant gains Monday, according to both sides.

Rebel forces in Luhansk said Monday that 30 people had died in clashes with government troops since the previous day and that Ukrainian forces had established a checkpoint within the city limits, Interfax reported.

Residents of Donetsk also spoke Monday of intense fighting around the city’s perimeter.

The escalation comes ahead of a European Union summit Wednesday at which leaders will decide whether to impose new sanctions on Russia over its role in the conflict, although the bloc’s 28 nations do not appear to be united on whether to push Russia any harder.

Demirjian reported from Moscow.