NOVOHANNIVKA, Ukraine -- Ukraine's new president Saturday declared today as 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 members 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 U.S., insists is supported by Russia.
The downing of the plane drew condemnation and concern from the White House, European leaders and U.N. General-Secretary Ban Ki-moon. Analysts said it could bring a renewed emphasis on increasing sanctions against Russia.
Ukrainian President Petro Poroshenko spoke firmly to glum-faced security officials at a televised emergency meeting Saturday, 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 members and 40 troops were aboard the Il-76 troop transport when it went down early Saturday 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.
Defense 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 Saturday in the southern port of Mariupol when their column of vehicles was ambushed, the guards service said.
The U.S. government reiterated its support for Mr. Poroshenko's government and rejected Russia's statements that it was not arming the rebels. The U.S. 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.
The Kiev government has accused Russia of permitting three tanks to cross the border last 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 armored vehicle ventured about 150 yards Friday into Russia. The Russian Foreign Ministry warned Saturday if the incursions continued it would "take all necessary measures to suppress them."
NATO, meanwhile, released images Saturday that it said showed recent Russian tank movements near the border. It said the tanks seen in eastern Ukraine "do not bear markings or camouflage paint like those used by the Ukrainian military." It said those tactics were used by the Russians who had seized Crimea in March.