Ukraine's new president 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 battle against an armed insurgency that the government, backed by the U.S., says is supported by Russia. It also came only a week after the new president, billionaire candy magnate Petro Poroshenko, spoke about a peace plan in his inaugural address.

The downing of the plane drew condemnation and concern from the White House, European leaders and UN chief Ban Ki-moon. 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 PLC. "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 Saturday, scolding the head of the country's SBU security service for "omissions" in measures to protect military aircraft.

The U.S. State Department says Secretary of State John Kerry has pressed his Russian counterpart to make clear Moscow's commitment to end the flow of weapons and other support to separatists in Ukraine.

The U.S. contends Russia has sent tanks and rocket launchers to the rebels.

Ukraine crisis

Remnants of a downed Ukrainian army aircraft Il-76 at the airport near Luhansk, Ukraine, on Saturday. Pro-Russian separatists shot down the military transport plane Saturday in the country's restive east, killing all 49 service personnel on board, Ukrainian officials said. (Evgeniy Maloletka/The Associated Press)

A State Department official says Kerry expressed "strong concern" about the shootdown and the flow of weapons and militants across the Russian-Ukraine border. Kerry also urged a fair settlement of a natural gas dispute.

Kerry also spoke with Ukraine's prime minister, Arseniy Yatsenyuk.

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.

Canada reacts

"On behalf of all Canadians, I extend my condolences to the families and friends of those who lost their lives. Canada also strongly condemns the incursion into Ukrainian sovereign territory of tanks and armoured personnel carriers flying the Russian flag, and calls for Ukraine's territorial integrity to be respected."


-Statement from Canadian Minister of Foreign Affairs John Baird

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.

Nine crew and 40 troops were aboard the Il-76 when it went down early Saturday as it approached the airport at the city of Luhansk, the Ukrainian prosecutor general's office said in a statement.

That exceeded the loss of 12 soldiers including a general on May 29 when rebels shot down a troop helicopter near the eastern city of Slovyansk.

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.

Source of rebel military gear questioned

The incident underlines questions about rebel access to military gear. Ukraine has accused Russia of permitting three tanks to cross the border where they were used by rebels. Russia denies supplying the separatists.

NATO released images on Saturday, said to show recent Russian tank movements near the border which "raise significant questions" on Russia's role.

Ukraine crisis

Ukraine has begun a ceasefire that would permit pro-Russian forces to leave the country. (Andrei Petrov/The Associated Press)

The tanks seen in Ukraine, NATO said, "do not bear markings or camouflage paint like those used by the Ukrainian military. In fact, they do not have markings at all, which is reminiscent of tactics used by Russian elements that were involved in destabilizing Crimea."

Denis Pushilin, a leader of the separatist Donetsk People's Republic, told Russian state television Friday that rebels had the tanks but it was "improper to ask" where they got them.

The defence ministry's statement said that the rebels "cynically and treacherously" downed the plane using anti-aircraft guns and heavy machine-guns. It expressed sympathy to the families of those killed "for their tragic and irreparable loss."

Alexei Toporov, defence spokesman for the self-proclaimed Luhansk People's Republic, said the aircraft was shot down after what he termed Ukrainian "occupiers" refused an ultimatum to abandon the Luhansk airport.

Luhansk is in Ukraine's east near the border with Russia, an area that has seen separatists seize government buildings and declare independence after holding disputed referendums.

Ukraine retakes some Mariupol buildings

Ukraine had claimed a success on Friday when troops retook some rebel-occupied buildings in the port city of Mariupol. No deaths were reported.

Ukraine crisis

Ukrainian government forces reclaimed the city of Mariupol from pro-Russian separatists in heavy fighting, and said they had regained control of a long stretch of the border with Russia. (Osman Karimov/Reuters)

Before Saturday's incident, the Ukrainian health ministry had said at least 270 people had died in clashes between government forces and armed separatists.

Tensions between Ukraine and Russia escalated in February after pro-Russian president Viktor Yanukovych was driven from office by a massive protest movement made up of people who want closer ties with the European Union.

Another official, counter-terrorism spokesman Vladislav Seleznyov, was quoted by the Interfax news agency as saying the death toll was preliminary and was being verified by investigators.

The Russian-built Il-76 is a four-engine jet that can be used to transport heavy gear and people.

The prosecutor's office said a criminal investigation had been opened under anti-terrorism laws.

Also in Kyiv, 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."

With files from CBC News