Pro-Russian separatists in eastern Ukraine have claimed responsibility for shooting down a Ukrainian military transport plane, and say four crew members have been taken prisoner.
Russia's Interfax news agency on Monday quoted separatists in the rebel-held city of Luhansk as saying the captives are in the border city of Krasnodon and undergoing interrogation.
Details of the shootdown were not clear late Monday. Interfax quoted the Ukrainian Defense Ministry as saying eight people were aboard the aircraft, which was carrying food and drinking water to personnel at checkpoints in the war-scarred east.
The ministry also said the plane was flying too high above Luhansk to have been hit by conventional surface-to-air missiles, prompting speculation that rebels either used more powerful Russian missiles, or that the plane was hit by Russian missiles from Russian territory
Ukraine on Monday accused Russian army officers of fighting alongside separatists in the country’s east.
“Information has ... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces,” President Petro Poroshenko said.
Poroshenko voiced similar complaints about Russian incursions on Sunday to the European Union presumably in hopes of pushing the bloc to exert greater pressure on Moscow in the form of expanded sanctions.
His office said Kyiv would provide documents offering proof of incursions from Russia to the international community through diplomatic channels.
Moscow has been denying that is supporting the rebels with personnel or equipment.
Reuters video on Monday also showed Ukrainian armored vehicles destroyed in an attack near Luhansk.
After a campaign to break through the perimeter of Luhansk over the weekend, Ukraine's Defense Ministry claims government troops are in control of villages around the rebel-held city of about 428,000 people.
Locator map of Luhansk, Ukraine
Meeting with security chiefs on Monday, Ukrainian President Petro Poroshenko accused Russian military officers of fighting alongside rebels to strengthen separatist forces in the country's east. He added that a new Russian missile system was being used against Ukraine forces.
The latest Ukrainian push to regain control of the Russian-speaking east came as Moscow blamed its neighbor for the cross-border artillery shelling death of a Russian civilian.
The Kremlin threatened Ukraine with "irreversible consequences" for the death, which Moscow said came from an errant artillery shell. Ukraine has denied involvement, and said the shelling may have been the work of provocateurs seeking to trigger a Russian response.
Renewed Russian troop build-up?
Meanwhile, Ukraine has again accused Russia of a renewed troop build-up along its border, with separatists backed by “mercenaries” firing on Ukrainian border guards in attempts to resupply fighters with equipment from Russia, a Ukrainian security spokesman said.
Saying that Russia was embarking on a course of escalation in Ukraine's east, National Security and Defense Council spokesman Andriy Lysenko told journalists: "In the past 24 hours, deployment of [Russian] units and military equipment across the border from the Sumy and Luhansk border points was noticed. The Russian Federation continues to build up troops on the border.''
"The events of the last few days show that the Russian side has embarked on a course of escalation of the conflict in the Donbas,'' he said, using the Ukrainian name for the industrial heartland in the country’s east.
Meanwhile, NATO is confirming that Russia has continued to build up troops along its border with Ukraine. According to current alliance assessments, Russia now has between 10,000 and 12,000 troops in the area, up from below 1,000 in mid-June, a NATO official told VOA.
Forces include a wide range of combat capabilities, including infantry, armored forces and tanks, artillery, logistics and supply personnel and vehicles, as well as special forces, the official said.
Fighting has surged in the east since the Ukrainian government refused to renew a unilateral cease-fire June 30. Since then, Ukrainian forces have driven pro-Moscow rebels out of several cities, including Slovyansk, which was their main stronghold.
Kyiv and many Western countries accuse Russia of fueling the insurgency in an effort to destabilize Ukraine following the ouster of its pro-Moscow president and Kyiv’s pursuit of a pro-Western course.
VOA's Jeff Seldin contributed to this report. Some information provided by Reuters.