Ukrainian President Petro Poroshenko said on January 22 that pro-Russian rebels have paid a "high price" for their escalating attacks on government troops across the separatist-controlled east.
Poroshenko made the statement at an emergency security meeting convened on January 22 after Kyiv's decision to give up its months-long defense of Donetsk airport.
His office quoted him as saying, "Our enemy will pay a high price for its opportunistic attacks on our armed forces."
Meanwhile, pro-Russian rebels on January 22 paraded captured Ukrainian soldiers at a bus stop in the separatist stronghold of Donetsk where an explosion killed at least seven civilians earlier in the day.
The carnage came a day after 16 wounded Ukrainian soldiers were captured amid the government’s withdrawal from key positions at the Donetsk airport.
Reports say residents hurled glass and shouted abuse at the captive Ukrainian soldiers, forcing them to their knees at a bus stop where the explosion struck a trolleybus.
Video footage showed a purported Ukrainian soldier with his hands tied behind his back being walked past one dead body and brought before a group of reporters at the site of the attack.
A group of angry civilians then beat the prisoner before he was pushed into a vehicle by masked gunmen.
OSCE monitors who visited the bus stop earlier in the day said they saw seven bodies there.
Separatists and medical workers say 13 people were killed.
Moscow and Kyiv traded accusations over the bus stop blast, which witnesses said was caused by a mortar or artillery shell.
WATCH: Deadly blast at Donetsk bus stop
Prime Minister Arseniy Yatsenyuk, speaking at a Kyiv ceremony marking Ukraine’s National Unity Day, blamed pro-Russian separatists and said Russia should bear responsibility.
Russian Foreign Minister Sergei Lavrov described the attack as a "monstrous new crime" by Ukrainian forces and a "crude provocation" aimed at undermining peace efforts.
Kyiv says the 16 injured Ukrainian soldiers were captured late on January 21 when government forces withdrew from the ruins of the Donetsk airport’s new terminal and from positions near the old terminal building.
Both terminals have been the scene of a fierce battle for months.
Fighting continued at the airport on January 22, with Kyiv saying its troops still held parts of the facility.
Ukrainian military spokesman Vladyslav Seleznyov said the decision was made to withdraw 20 Ukrainian soldiers because “their positions had been destroyed and exposed to direct fire,” leading to the deaths of six soldiers during the previous 24 hours.
He said the new terminal building “looks like a sieve and there’s simply nowhere to hide there.”
In the last week, about 100 wounded government soldiers were evacuated from the airport’s new terminal building.
The latest violence has heightened the challenges faced by diplomats who are trying to negotiate an end the conflict in eastern Ukraine.
Interfax quoted a Donetsk separatist leader, Denis Pushilin, as saying on January 22 that "any strike launched against civilian areas and civilian casualties will only serve to complicate the negotiating process."
Amnesty International said the blast is “likely to be a violation of international humanitarian law and must be investigated promptly and impartially.”
Lavrov called for a complete and impartial investigation by the OSCE monitoring mission.
The blast at the bus stop came hours after a meeting in Berlin that German Foreign Minister Frank-Walter Steinmeier said had produced an agreement for the rebels and government forces to pull back heavy weaponry 15 kilometers from a demarcation line established under a cease-fire deal agreed in Minsk in September.
Steinmeier said the "contact group" -- representatives of Ukraine, Russia, and the OSCE -- would be meeting in the coming days to work out the details and timeline for the weapons pullback.
He said the agreement represented progress but "no breakthrough" in efforts to end the conflict, which has driven ties between Moscow and the West to a state of tension unseen since the Cold War.
Meanwhile, Ukrainian President Petro Poroshenko said in a televised statement on January 22 that Ukraine will remain a "unitary" state with strong central power in Kyiv and that its pursuit of integration with Europe must not be questioned.
His remarks on Ukraine’s National Unity Day sent a defiant message to Moscow in the midst of the conflict, a day after he accused the Kremlin of deploying more than 9,000 Russian troops along with 500 tanks, armored personnel carriers, and heavy artillery pieces in eastern Ukraine.
The OSCE said on January 22 that more than 5,000 people have been killed since April in the conflict between the separatists and Ukrainian government forces.
NATO says it intends to reestablish contact with Russian military leaders after months of tension over the conflict in eastern Ukraine.
U.S. General Philip Breedlove, NATO's top military commander, said in Brussels on January 22 that alliance officials have discussed how to "reestablish [communication] and the fact that the communication with our senior military interlocutors in Russia is important."
He said several senior NATO military leaders had decided to renew communication with Russian General Valery Gerasimov, chief of the Russian armed forces general staff.
Breedlove said he had spoken to Gerasimov even while Russian troops advanced into Ukraine's Crimean Peninsula early last year.
With reporting by Reuters, AP, AFP, dpa, Interfax, and TASS