The Ukrainian military has launched air strikes against separatist strongholds in the country's eastern region following the shooting-down of a transport plane that killed 49 people.
The military on Saturday targeted rebel checkpoints in Luhansk and a military base that had been taken over by pro-Russian separatists, in retaliation for the downing of the plane.
"It's a very dangerous development here," Al Jazeera's Kim Vinnell, reporting near Luhansk, said. "Separatists in Luhansk are very much in control of the city, and they say that they are going to meet this attack with everything that they have got."
Al Jazeera's Neave Barker, reporting from the city of Horlivka, said shelling could also be heard in the nearby city of Donetsk, which is also controlled by the rebels.
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.
Overnight, rebel fighters, using anti-aircraft weapons and a heavy-calibre machine gun, brought down the plane as it was coming in to land, a defence spokesman said.
Charred debris was scattered for hundreds of metres over the sloping wheat field where the plane came down near Novohannivka, a village 20km southeast of Luhansk.
The tail section jutted up from the ground, with parts of the engines, fuselage and other parts lying around it.
A platoon of rebel forces clad in camouflage fatigues scoured through the ruins for ammunition that had been intended for the government forces in east Ukraine.
Rebel fighters said they fired on the plane after it violated a "no-fly zone" imposed against government aircraft.
The toll is the highest suffered by government forces in a single incident since the conflict in Ukraine began, and is likely to fuel tension between Russia and Kiev's main ally, the United States, which accuses Moscow of arming the rebels.
In Kiev, the deadly plane attack has set off a protest outside the Russian embassy, with demonstrators turning upside-down vehicles reportedly owned by embassy staff.
Meanwhile, NATO on Saturday released satellite pictures that it said raised suspicions about Russia's role in moving military equipment into eastern Ukraine.
NATO said that the images showed Russian tanks arrived at a staging area close to the Ukraine border days before similar tanks appeared this week in areas of eastern Ukraine under separatist control.
Ukrainian Interior Minister Arseny Avakov said the tanks had rolled across the border from Russia. Rebels claimed they were from a Ukrainian military stockpile.
Evidence that Russia is sending in heavy armour and weapons could encourage the US and the EU to impose new sanctions on Moscow, so far limited largely to visa bans and asset freezes on some individuals, banks and companies.