In the biggest single loss of life since Ukraine started battling pro-Russian separatists, rebels shot down a military transport plane early Saturday morning as it was landing in Luhansk, killing all 49 people aboard.

Rebels using an anti-aircraft weapon and large-caliber machine gun downed the Ilyushin-76, according to a statement by the Defense Ministry. The statement said the aircraft was carrying equipment and food as well as military personnel who were being flown in as part of a routine rotation, and the plane was approaching the city airport in Luhansk, a separatist stronghold. While the rebels control most of the city, the army has control of the airport.

The deadly attack on the plane was certain to further escalate tensions between Moscow and Kiev, which has accused Russia of aiding the separatists by sending them heavy military equipment, a charge Russia has denied.

On Friday, the U.S. State Department confirmed Kiev’s suspicions, saying Russia has been sending the separatists military equipment, including tanks and rocket launchers.

U.S. State Department spokesperson Marie Harf said a convoy of three Russian tanks, several Grad rocket launchers and other military vehicles had crossed the border from Russia into eastern Ukraine over the previous three days.

NATO Secretary-General Anders Fogh Rasmussen said the deployment of Russian equipment into Kiev, if accurate, would be a “serious escalation” in the conflict.

He urged Russia “to complete the withdrawal of its military forces on the border with Ukraine, to stop the flow of weapons and fighters across the border, and to exercise its influence among armed separatist to lay down their weapons and renounce violence.”

Ukrainian military officials said the plane was carrying 40 soldiers and a crew of nine when it was shot down around 1:10 a.m.

In its statement, the Defense Ministry statement blamed “terrorists,” who it said had “cynically and treacherously” fired on the plane.

Military spokesman Vladislav Seleznev said all 49 military personnel had died, and a recovery operation was underway Saturday morning.

Luhansk is the biggest city in one of two breakaway regions in eastern Ukraine where separatists have declared People’s Republics and want to join the Russian Federation. Hundreds of people have been killed in the fighting that has engulfed the region since April, and tens of thousands of civilians have fled the region for safety.

The fighting has intensified in recent weeks since the Ukrainian military launched an offensive against the insurgents after the May 25 election of President Petro Poroshenko.

On Friday, the government scored a significant victory when the army ousted rebels from the town of Mariupol, regaining control over a stretch of the border with Russia.

The pre-dawn attack on separatist strongholds in Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, Interior Minister Arsen Avakov said. At least five separatists were reported killed and four Ukrainian soldiers were wounded.

Mariupol is considered strategically important because it is situated on major roads and steel is exported through its port. Separatists have infiltrated Mariupol several times during the conflict, and full Ukrainian control may prove to be only temporary. But in a sign that Ukrainians expect to stay in charge, President Petro Poroshenko ordered Serhiy Taruta, the Donetsk governor who has been ruling from Kiev in recent weeks, to relocate immediately to Mariupol.

After a series of setbacks, the battlefield victory underscores a growing confidence among Ukrainian officials that the tide may have turned in the conflict that has raged since April.

Poroshenko has called on the separatists to lay down their arms and said he will negotiate with anyone who does not have the blood of innocents on his hands. At the same time, the army has been increasingly aggressive in attacking separatists, who control a swath of the Donetsk and Luhansk regions, where they have proclaimed “people’s republics” and asked for international recognition from several countries, such as Russia and Venezuela.

Avakov also said Friday that government troops again control a 75-mile stretch of the 1,250-mile border that had been in the hands of insurgents.

An aide, Anton Gerashchenko, predicted that the entire border, “which remained naked after massive terrorist attacks,” would be under Ukrainian control by this weekend.

“If they attempt to infiltrate, the militants will be destroyed,” he said.

The border, which was considered porous even before the conflict began, has been easily crossed by fighters and convoys bearing supplies. Moscow sent a note of protest to Kiev, saying two Ukrainian armored vehicles had crossed over the border in Russia’s southern Rostov region early Friday.

On Wednesday, three seemingly old and decommissioned tanks rumbled through the Donetsk and Luhansk regions, although their exact origin is murky. Ukrainian officials said they came across the border from Russia, but Moscow denied that they were ever on Russian territory. Whatever their provenance, rebel leader Denis Pushilin, who escaped an apparent assassination attempt Thursday, told Russian state television Friday that the tanks are in the hands of the Donetsk People’s Republic, although he deflected questions about how they were obtained.

Some Ukrainian officials are looking for ways to fortify border security once the conflict is under control. Igor Kolomoisky, a billionaire who was appointed governor of the region around Dnipropetrovsk, has urged Poroshenko to begin building an impermeable fence along the border in the regions of Donetsk, Luhansk and Kharkiv.

He compared such a fence to the Mannerheim Line, which Finland erected along its border with the Soviet Union in the 1920s as a bulwark against the Communists who came to power in the Bolshevik Revolution. He also noted that Israel has built a wall separating it from Palestinians in the West Bank.

Kolomoisky apparently has given the idea of a Ukrainian wall some thought. He proposed that an electrified fence be constructed of high-strength steel topped by barbed wire and with moats on either side to deter civilians and animals. He estimated that the fence would cost the cash-strapped state at least $70 million, or perhaps double that.

But first, Ukraine must regain control of the breakaway regions.

Government accounts of Friday’s battle to retake Mariupol, if accurate, would suggest that not all the insurgents are ferociously committed to their cause.

Gerashchenko told reporters that about 30 separatist fighters were detained. Many were hiding in buildings and basements, he said, and were warned that if they did not surrender, troops would hurl in hand grenades.

He said the captured fighters provided details about the locations of mines and sniper lookouts. Security forces were able to defuse two bombs in one building before they exploded, he said.

By midmorning, yellow-and-blue Ukrainian flags once again were flying from the city administration building and the city council building.

“The city returns to its normal life,” Gerashchenko said.