The Ukrainian army retreated Thursday from key strongholds at the Donetsk airport, an epicenter of fighting in the country’s conflict-battered eastern region, handing a symbolic victory to pro-Russian rebels amid a surge of violence that threatens to further unravel peace efforts.

Tensions in Ukraine have ­escalated since the start of the new year to levels that NATO’s top commander said he has not seen since summer, before ­government troops and ­pro-Russian rebels signed a cease-fire agreement — an accord rendered ineffective by the recent surge in violence. Ukraine’s Western-allied government has also leveled new charges that Russia has sent forces over the border to bolster the insurgents.

Moscow denied the claims and traded accusations with Ukraine on Thursday over which side was responsible for the deaths of at least 13 people at a Donetsk bus stop that was hit by a mortar shell early Thursday.

The bloody developments came as Russia, Ukraine, Germany and France held negotiations Wednesday night in Berlin, where leaders reported they had agreed on the line from which Ukrainian troops and pro-Russian rebels would withdraw their heavy weapons.

Ukraine and its allies charge that the rebels’ recent moves are aimed at scooping up extra territory.

This video, shot on Jan. 15, was published by a fundraising collective for the Ukrainian army that calls itself Army SOS and gives a bird’s-eye-view of the badly damaged airport in Donetsk, Ukraine. (Yury Kasyanov/Army SOS/YouTube)

U.S. Secretary of State John F. Kerry accused “Russian-backed separatists” Wednesday of engaging in “a very blatant land grab” that violates the terms of the cease-fire agreement struck in Minsk, Belarus, in September.

U.S. Air Force Gen. Philip M. Breedlove, NATO’s top commander, told reporters Thursday that “Russian-backed forces” had “moved the line of contact to the west.” He also said NATO had observed markers of air and electronic weapons systems “that have accompanied past Russian troop movements into Ukraine.” But he could not yet put a figure on how many Russian weapons and troops had recently crossed the border. [Related: Ukraine parliament votes to take step toward NATO.]

On Wednesday, Ukrainian President Petro Poroshenko told the World Economic Forum in Davos, Switzerland, that Russia has sent 9,000 troops into Ukraine.

The Ukrainian military, meanwhile, described its retreat from the Donetsk airport’s new terminal as a tactical move — not an admission of defeat.

“This is a battlefield at the moment,” Col. Andriy Lysenko, spokesman for Ukraine’s National Security and Defense Council, told reporters Thursday. He said that while the military still controlled some smaller buildings at the airport, the facility was so damaged it was no longer worth defending. “Our military is not withdrawing, it is not retreating.”

The terminal was constructed ahead of the Euro­­ 2012 soccer championship, but it has been largely destroyed during months of fighting.

(Related:Ukraine parliament votes to take step toward NATO)

Several people were killed in an attack on a bus Thursday morning in the rebel-controlled city of Donetsk in eastern Ukraine. Windows of shops nearby were blown out by the blast. (Reuters)

[Related: Russia’s response to Obama: ‘U.S. intends to dominate the world.’]

(Related: Russia’s response to Obama: ‘U.S. intends to dominate the world’)

But the commander of the Azov battalion, a volunteer unit of the Ukrainian army at the Donetsk airport, declared the airport “lost” on the battalion’s Facebook page.

“The heroic defense continued for 242 days,” the battalion said. “That is longer than the defense of Stalingrad and Moscow during the Soviet-German war; it is longer than a whole war can last, sometimes.”

The loss of the airport would be a symbolic victory for the rebels, who have made securing the site a key goal.

The Ukrainian military counted 10 soldiers killed in the past day’s clashes Thursday, at least six of them in the airport fighting. Ukrainian government negotiator Yuriy Tandit later told television station 112 Ukraine that more than 20 others had been captured by pro-Russian rebels. At least one captured soldier was taken to the scene of the bus stop shelling in Donetsk and paraded before angry residents on Russian state television.

Alexander Zakharchenko, head of the self-proclaimed ­Donetsk People’s Republic, told reporters that the bus stop was struck by artillery fire coming from Avdiivka, a town in the Donetsk region where the Ukrainian military said it has been attacked by pro-
Russian rebels, according to Russian news service Interfax. He rebutted earlier reports citing pro-Russian rebels who described the shelling as an “act of sabotage.”

Ukrainian officials blamed the rebels for the attack on the bus stop. Ukrainian Prime Minister Arseniy Yatsenyuk said that “the responsibility for it should be borne by the Russian Federation.”

It was not immediately clear whether the site was hit by a single shell or part of a barrage.

The separatist-run Donetsk News Agency cited officials as saying the shell was fired from a portable 82mm mortar. That type of weapon suggested the fire came from rebel-held territory, the Associated Press reported, but there was no confirmed location for the source of the shell.

Russian Foreign Minister Sergei Lavrov called for an independent investigation of the attack but blamed Kiev “and its foreign patrons.” He charged that the attack was a “crime against humanity” and a “gross provocation aimed at undermining efforts at a peaceful settlement of the Ukrainian ­crisis.”

But German Chancellor Angela Merkel — widely seen as Russia’s closest European negotiating partner — said at Davos on Thursday that Russia’s actions in Ukraine, especially its annexation of Crimea in March, were a “clear and flagrant violation” of Europe’s peaceful postwar order.

Noting that economic sanctions against Russia could be lifted if there is progress toward peace, Merkel said: “We’re not there yet.”

Russia’s relations with NATO appear to be thawing slightly, however. On Thursday, Breedlove told reporters that NATO intends to “reestablish” dialogue with Russian counterparts soon.