A Ukrainian military plane was shot down by rebels using an anti-aircraft missile Saturday, killing all 49 personnel on board and providing further evidence of an increasingly well-armed pro-Russian rebellion.
The attack prompted a retaliatory reponse from Kiev, with air strikes launched against separatist strongholds in the country's restive east.
The military targeted rebel checkpoints in Luhansk and a base that had been taken over by pro-Russian separatists. Shelling could also be heard in the nearby city of Donetsk, which was also controlled by the rebels.
On Saturday, Ukrainian President Petro Poroshenko summoned his security chiefs for consultations and promised an "adequate" response after the transport plane was shot down.
"All those involved in cynical acts of terrorism of this magnitude must be punished," he said, declaring Sunday a day of mourning for the nine crew and 40 paratroopers aboard the Ilyushin-76 aircraft.
Ukraine's military response came just hours after the plane was shot down as it approached the airport outside Luhansk, the Ukrainian prosecutor general's office said in a statement. On Saturday, armed separatists were seen inspecting the scorched wreckage in a field 12 miles south of Luhansk.
The incident is the latest that has raised concerns among Ukrainians and Western leaders about how rebels are gaining access to military equipment. Ukraine on Friday accused Russia of permitting three tanks to cross the border into east Ukraine to be used by rebels.
On Friday, the U.S. State Department said it was “confident” that Russia was supplying military aid to the rebels.
“We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers,” Marie Harf, deputy spokeswoman for the State Department, said in a statement.
Moscow denies supplying the separatists, but on Saturday NATO released images said to show recent Russian tank movements near the border that "raise significant questions" on Russia's role.
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.
Any evidence that Russia is sending in heavy armor and weapons could encourage the U.S. and the EU to impose new sanctions on Moscow. But to date, the penalties — so far limited largely to visa bans and asset freezes on some individuals, banks and companies — have seemingly had little to no impact on Moscow policy toward its neighbor.
The Ukrainian defense ministry's statement said in regards to the latest incident that the rebels "cynically and treacherously" downed the transport 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, defense spokesman for the self-proclaimed Luhansk People's Republic, said the aircraft was shot down after the 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 over the last several months.
Ukraine had claimed a success on Friday when troops retook some rebel-occupied buildings in the port city of Mariupol. No deaths were reported.
Before Saturday's incident, the Ukrainian health ministry had said at least 270 people had died in clashes between government forces and armed separatists.
Saturday's events led to strong rebukes of Russia from the West.
U.S. Secretary of State John Kerry held a call with Russian Foreign Minister Sergey Lavrov in which Kerry expressed "strong concern" about the recent events in Ukraine. Kerry warned that the U.S. and other countries in the G7 were ready to "raise costs for Russia," if the country continued to send weapons across the border to Ukraine, according to a White House statement.
Tensions between Ukraine and Russia escalated in February after pro-Russian President Viktor Yanukovich was driven from office by a massive protest movement made up of people who want closer ties with the European Union.
Al Jazeera and wire services