Ukrainian President Petro Poroshenko has declared a day of mourning for those killed in the deadliest attack on national troops since the pro-Russian uprising began in the country's south and east.
Officials said 49 people -- 40 troops and nine crew members -- perished when a Ukrainian army transport aircraft was struck by a surface-to-air missile in the early morning hours of June 14 as it approached Luhansk airport, where heavily armed separatists control much of the area.
Poroshenko said in his statement that the day of mourning, June 15, marks "a great loss not only for the families of the victims but also for the whole country. Ukraine is in sorrow, but we strongly continue the struggle for peace."
The apparent escalation in fighting comes after weeks of images of Russian gunmen crossing the border and amid fresh allegations by the United States that "separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers."
"All those involved in the cynical terrorist attack of such magnitude must be punished," says the statement by Poroshenko, who vowed after his election in May and again at his June 7 inauguration that talks in the separatist east and with Moscow were a priority. "Ukraine needs peace. Counterterrorism receives an adequate response."
Ukraine’s Defense Ministry said the 49 soldiers died overnight when pro-Russian separatist fighters in eastern Ukraine shot down the IL-76 transport plane as it approached Luhansk airport.
The airport has been under the control of government forces but separatist fighters hold most of the rest of the city.
Meanwhile, separatists said they also shot down a Ukrainian air force bomber after it hit a target in the rebel-held town of Horlivka, in Donetsk region.
Kyiv is continuing a major military operation to defeat the insurgency in eastern Ukraine.
Warning From Washington
The downing of the army aircraft came just hours after Washington said it could confirm Kyiv’s claims that Russia has been sending heavy military equipment to pro-Russia separatists in eastern Ukraine -- including tanks and rocket launchers.
"We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers," U.S. State Department spokeswoman Marie Harf said.
She said a convoy of three T-64 tanks, several MB-21, “or Grad” multiple rocket launchers, and other military vehicles had crossed from Russia into eastern Ukraine “near the Ukrainian town of Snizhne” during the last three days -- moving through several towns, including Snizhne, Torez, and Makiyivka.
Harf said: "Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank units have been operating in that area. We are confident that these tanks came from Russia."
She also said Washington has "information that Russia has accumulated multiple rocket launchers at this same deployment site in southwest Russia, and these rocket launchers also recently departed"
She said Internet video has shown what Washington believes to be "these same rocket launchers traveling through Luhansk."
NATO Secretary-General Anders Fogh Rasmussen said earlier on June 13 that if the reported deployment was confirmed, it would mark a “serious escalation” of the crisis.
He urged Russia “to complete the withdrawal of its military forces on the border with Ukraine, to stop the flow of weapons and fighters across the border, and to exercise its influence among armed separatist to lay down their weapons and renounce violence.”
Rasmussen's remarks came after Kyiv accused Russia of allowing pro-Russian separatists who are fighting Ukrainian government forces to transport three tanks and other military vehicles across the border into eastern Ukraine.
President Poroshenko told Russian President Vladimir Putin in their first official telephone conversation on June 12 that the situation was "unacceptable."
Denis Pushilin, a pro-Russia separatist leader in Donetsk, told Russian state television on June 13 that separatist had obtained tanks but it was “improper to ask” where they had gotten them.
Pushilin said the tanks were “in Donetsk and are the minimum that we have to defend the city.”
The developments come as the Ukrainian government continues a military operation to reclaim territory from pro-Russian separatists in eastern Ukraine.
Ukraine’s Interior Minister Arsen Avakov said on June 13 that government troops had regained control from separatists of the southeastern port city of Mariupol.
But an ambush on a vehicle carrying border guards early on June 14 in Mariupol reportedly killed at least three people.
Avakov said government forces also won back control of a 120-kilometer stretch of the border with Russia that had fallen to the rebels.
Meanwhile, Putin's spokesman said the Russian president has ordered Russia's Foreign Ministry to issue a formal complaint with Kyiv after a Ukrainian armored vehicle was found on Russian territory, close to the border with Ukraine's Luhansk region.
Russian border guards said the Ukrainian armored personnel carrier was found about 150 meters within Russian territory.
With reporting by AP, Reuters, AFP, CNN-TV, ITAR-TASS and Interfax