Ukraine's new president has declared Sunday a day of mourning and vowed to punish those responsible after pro-Russia separatists shot down a Ukrainian military transport plane, killing all 49 crew and troops aboard.
It was a bitter setback for the Ukrainian forces - the deadliest single incident yet in their escalating battle against an armed insurgency that the government, backed by the US, insists is supported by Russia.
The downing of the plane drew condemnation and concern from the White House and European leaders. Analysts said it could bring a renewed emphasis on increasing sanctions against Russia.
"(This) will refocus attention on the fact that Russia does not seem to be doing very much to moderate the insurgency (or) the cross-border resupply of separatists," said Timothy Ash, an analyst at Standard Bank. "I would expect the focus to return to sanctions next week."
Ukrainian president Petro Poroshenko spoke firmly to glum-faced security officials at a televised emergency meeting, scolding the head of the country's SBU security service for "omissions" in measures to protect military aircraft.
Mr Poroshenko called for "a detailed analysis of the reasons" for the lapse and hinted that personnel changes were imminent. His office said he vowed to punish "those responsible for the tragedy in Luhansk".
In a conversation with French president Francois Hollande, Mr Poroshenko expressed hope that the European Union would decide on further sanctions against Russia if what he called the illegal border crossings and the supply of weapons did not cease.
Nine crew and 40 troops were aboard the Il-76 troop transport when it went down as it approached the airport at Luhansk, the Ukrainian prosecutor general's office said.
The plane's tail section and other pieces of scorched wreckage lay in a field near the village of Novohannivka, 12 miles south of Luhansk. An Associated Press reporter saw a dozen or more armed separatists inspecting the crash site.
Defence Ministry spokesman Bohdan Senyk said the rebels used anti-aircraft guns and a heavy machine gun to down the plane, while the prosecutor general's office said rebels used an anti-aircraft missile.
Luhansk, a city near the border with Russia, is one of two eastern areas where separatists have seized government buildings and declared independence. Ukrainian forces still control the Luhansk airport.
In other fighting, five border guards were killed and seven wounded in the southern port of Mariupol when their column of vehicles was ambushed, the guards service said.
The US government reiterated its support for Poroshenko's government and rejected Russia's statements that it was not arming the rebels. The US said Russia had sent tanks and rocket launchers to the rebels, making sure the unmarked tanks were of a type not currently being used by Russian forces.
"We condemn the shooting down of the Ukrainian military plane and continue to be deeply concerned about the situation in eastern Ukraine, including by the fact that militant and separatist groups have received heavy weapons from Russia, including tanks, which is a significant escalation," said White House spokeswoman Laura Lucas Magnuson.
Mr Hollande and German chancellor Angela Merkel jointly called Russian president Vladimir Putin to express their "dismay" over the downing of the plane and said the incident makes clear how urgent a ceasefire is, German government spokesman Georg Streiter said.
Ms Merkel stressed that, for a ceasefire to last, Russia must better control its border with Ukraine to stem the flow of weapons and fighters and the Russian government must also exert its influence on pro-Russia rebels.
The Kiev government has accused Russia of permitting three tanks to cross the border this week into eastern Ukraine, where they were used by rebels. Russia denies supplying the separatists and says Russians fighting in Ukraine are volunteers.
Moscow did not respond to the tank reports but instead accused the Ukrainian military of violating the border several times, including when an armoured vehicle ventured about 150 metres into Russia. The Russian Foreign Ministry warned if the incursions continued it would "take all necessary measures to suppress them".