Ukrainian President Petro Poroshenko summoned his defense and security chiefs for consultations Saturday after pro-Russian separatists shot down a military transport plane, killing all 49 people on board.
In a statement released by his office, Poroshenko said all those involved in "cynical acts of terrorism" must be punished.
"Ukraine needs peace. However, the terrorists will receive an adequate response," he said.
The prosecutor general's office said 40 service members and nine crew were killed early Saturday when the rebels shot down the Ukrainian Air Force Ilyushin-76 in eastern Ukraine.
The aircraft filled with troops and supplies was about to land at the Lugansk airport when it was fired on with anti-aircraft and heavy-caliber gunfire, the Defense Ministry said.
Other sources, including the widely-followed Ukrainian defense analyst Dmitry Tymchuk, said Russian-designed anti-aircraft missiles known as "Iglas" had been found in the area, and were likely used against the plane.
NATO on Saturday released photos that it said showed recent Russian tank movements near the Ukrainian border, a day after the U.S. State Department asserted that Russian tanks and other heavy weapons had crossed the border with Ukraine.
State Department spokeswoman Marie Harf told reporters Friday that a convoy of three T-64 tanks, several rocket launchers, and other military vehicles crossed the border near the Ukrainian town of Snizhne in the last three days.
She called the move "unacceptable."
Earlier Friday, NATO Secretary-General Anders Fogh Rasmussen said that if reports of "pro-Russian armed gangs" acquiring heavy weapons from Russia were confirmed, it would mark "a serious escalation" of the crisis in eastern Ukraine.
Russia, which annexed Ukraine's Crimea peninsula in March, has repeatedly said it would not make further intrusions into Ukrainian territory.
Meanwhile, Ukraine's government said that its security forces regained control of Mariupol, a port city in the Donetsk region that has changed hands several times.
Interior Minister Arsen Avakov said Ukrainian forces raised the country's flag over city hall.
Earlier, Ukrainian presidential bodyguards reportedly disposed of a powerful explosive device found near Poroshenko's headquarters, but the presidential administration did not immediately issue any statement.
A security source said on Saturday the device was found during the night near the gate where presidential cars go in.
"It was a container with five grenades and a kilogram of metal nuts," the unnamed official was quoted as telling Reuters. "It was a really powerful device."
Also Saturday, U.S. Secretary of State John Kerry pressed Russian Foreign Minister Sergei Lavrov to make clear Moscow's commitment to ending the flow of Russian weapons and tanks across the border into Ukraine.
Kerry also spoke by phone Saturday with Ukrainian Prime Minister Arseniy Yatsenyuk about an attack by protesters on the Russian Embassy in Kyiv. Footage of the incident shows people throwing eggs and paint at the embassy building and damaging and overturning cars while chanting anti-Russia slogans.