Ukrainian security officials said on Monday that contact had been lost with an An-26 military transport plane near Luhansk in east Ukraine where there is heavy fighting between Ukrainian forces and separatist rebels.
"Today, at around lunchtime, contact with this plane was lost. I cannot add anything more," Oleksiy Dmytrashkivsky, a spokesman for the government's "anti-terrorist operation", said.
Meanwhile, the city administration said at least three civilians were killed by shelling in Ukraine's eastern city of Luhansk on Sunday (July 13), as shocked residents assess the damage.
In the Luhansk southern district school a female guard has been killed by shrapnel and the building sustained serious damage, the school staff said.
"You are standing next to the School No. 51 of the southern quarter of Luhansk. As you can see walls are still standing after a massive attack, but the school sustained serious damage. A female guard at a musical school was killed, she didn't have enough time to hide in the basement," said Alexander Alpatov, a teacher from the School No. 51.
Ukrainian forces, taking the lead from President Petro Poroshenko, who swore to "find and destroy" the separatists who killed 23 servicemen in rocket strikes on Friday (July 11), went on the offensive across a broad range of targets south and south-east of the border town of Luhansk and near the city itself.
Poroshenko's office on Monday (July 14) said Ukrainian forces, backed by warplanes, had broken through rebel lines surrounding Luhansk airport, ending a separatist blockade.
President Poroshenko said Russian military staff officers were fighting against Ukrainian forces alongside separatist rebels and a new Russian missile system was being used on the separatist side.
"In the past three days a new Russian missile system has been used against the armed forces of Ukraine," he told security chiefs.
"Information has also been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces," he said, according to his website.
In turn, a Ukrainian military spokesman said Russia is building up forces on its border with Ukraine, and separatists, backed by Russian "mercenaries", are firing on Ukrainian border guards in an attempt to bring armored vehicles into the country.
Accusing Russia of embarking on a course of escalation in Ukraine's eastern regions, National and Security 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."
In the early hours of Monday, separatists had fired on border guards and the armed forces near the border settlement of Dyakove, one of several attacks on border guards as "terrorists and Russian mercenaries" tried to bring in armored vehicles and equipment, he said.
Lysenko accused rebel fighters of being behind the cross-border shelling of a Russian residential area in which a Russian man was killed and which Moscow says was the work of Ukrainian forces.
Russia was presenting these shooting incidents as the action of Ukrainian forces but these accusations were "absolutely unfounded," he said.
Also on Monday, Ukraine said its forces had ended a rebel blockade of a strategic airport in the east as it traded charges and threats with Russia over violations of their joint border during a weekend of fierce military combat.
Ukraine's military said its warplanes had inflicted heavy losses on the pro-Russian separatists during air strikes on their positions, including an armored convoy which Kiev said had crossed the border from Russia.
Ukrainian President Petro Poroshenko's office said Kiev would present documentary proof of incursions from Russia to the international community via diplomats on Monday.
REUTERS