Poroshenko called an emergency meeting of Ukraine's national security council and declared Sunday a day of national mourning.
Afterward, the president scolded the head of the country's SBU security service, referring to "omissions" in measures to protect military aircraft from attack. He called for "a detailed analysis of the reasons" and hinted that personnel changes were imminent.
In the southern port of Mariupol, five border guards were killed and seven wounded Saturday when their column of vehicles was ambushed, the guards service said.
The U.S. government reiterated its support for 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.
"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.
German Chancellor Angela Merkel and French President Francois Hollande called Russian President Vladimir Putin to express their "dismay" over the downing of the plane and said the incident makes clear how urgent a cease-fire is, German government spokesman Georg Streiter said in a statement.
Merkel stressed that, for a cease-fire to last, Russia must effectively 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.
Analysts said the downing of the plane could bring a renewed emphasis on increasing sanctions against Russia.
The incident "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 PLC.
"Comments from U.S. officials are now quite specific, and I would expect the focus to return to sanctions next week," he said.
Nine crew 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. 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.
The plane's tail section lay with other pieces of scorched wreckage in a field near the village of Novohannivka, 20 kilometers (12 miles) south of Luhansk. An AP reporter saw a dozen or more armed separatists inspecting the crash site.
The death toll Saturday exceeded the 46 who died after a fire and shootings in Odessa on May 2. At least 40 people also died in fighting at Donetsk airport in late May and a rebel spokesman said the toll on his side that day may have been as high as 100.
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 a Ukrainian armored vehicle ventured about 150 meters (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.
Tensions between Ukraine and Russia escalated in February after pro-Russian President Viktor Yanukovych was driven from office by protesters who wanted closer ties with the European Union and an end to the country's endemic corruption. Russia then seized and annexed Ukraine's Black Sea peninsula of Crimea.
The U.S. and Europe rejected the annexation and responded with financial sanctions targeting individuals. They have threatened to further extend the sanctions to the Russian economy.
The European Union, meanwhile, reported that senior officials from Ukraine and Russia, including Ukrainian Prime Minister Arseniy Yatsenyuk and Gazprom CEO Alexei Miller, were holding talks late Saturday in Kiev on their natural gas dispute.
Russia says Ukraine owes billions in unpaid gas debts and has set a deadline of Monday before it will demand upfront payments for gas supplies. Ukraine disputes the debt amount and, with its economy in dire straits after the departure of Yanukovych, has little ability to repay.
Also in Kiev, about a hundred protesters hurled eggs Saturday at the Russian Embassy and overturned several parked cars with diplomatic plates, holding a sign saying "Russia is a killer."
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