KYIV, Ukraine - In a town near the front with Russia in eastern Ukraine, grease-stained Ukrainian soldiers huddled over the engine hatch of a battle-damaged T-64BV battle tank. They had been working for three days straight.

"Fortunately, it wasn't a direct hit and the crew survived," said their commander, Zhenya, who for security reasons gave only his first name. "But the engine had to be replaced."

Inside this makeshift workshop near Ukraine's eastern front, soldiers in teams worked round-the-clock to repair the tanks and other vehicles that are the backbone of Ukraine's military resistance to Russia's invasion - Soviet-designed and -built equipment once destined for soldiers with allegiance to Moscow.

The Soviet Union, and later Russia, was a major arms exporter, seeding Eastern Europe in particular with tanks, artillery pieces, armored personnel carriers and air defense systems that are cheaper and simpler to use than U.S. weapons, experts said. With Eastern European countries sending equipment to Ukraine, that means Moscow has effectively armed its own enemy - one that was once at the heart of the Soviet Union's weapons production and innovation.

Now Ukraine has harnessed its Soviet-era inventory against Moscow. In a strange full-circle moment, for instance, Ukrainian hands built both the Moskva, the flagship of the Russian Black Sea fleet that sank this month off Odessa, and the missiles used to destroy it.

"Most of the equipment in the Ukrainian military at the beginning of the war was Soviet-era," said Mark Cancian, senior adviser with the Center for Strategic and International Studies. In addition to the tanks, Ukraine's aircraft and artillery originate from that period, he said - and artillery, in particular, has proved decisive in fighting the Russians since the war began.

Antitank weapons from the West, including Javelins from the United States, are perhaps the "most visible but not necessarily the most numerous" of the weapons at Ukraine's disposal, Cancian said.

"It's always awkward when you end up equipping both sides of a conflict," Cancian said.

The Ukrainians and other former Warsaw Pact countries still use many Soviet weapons, mostly because of existing stockpiles, said Jeremy Shapiro, director of research at the European Council on Foreign Relations.

Despite the vehicles' age, maintaining them and upgrading their components has greatly extended their service life. The armor and electronics may evolve, Shapiro said, "but the things that fire metal aren't that different."

The reliance on Soviet and Russian weaponry has at times made it a challenge for the United States and NATO partners to provide weapons Ukrainian troops need. For instance, U.S. artillery pieces provided to Ukraine use 155 mm rounds, which are incompatible with their howitzers. Instead of rushing munitions to the front, the United States must also send the guns, while also providing training to artillerymen in another country.

U.S. and Western officials are gathering in Germany to discuss the path forward in its defense relationship with Ukraine, including evaluating the willingness of Ukraine to be weaned off Russian equipment, U.S. officials have said.

"Transitioning from that equipment to NATO standard is something that will take years, even decades," Cancian said. "So this is not something that's going to happen overnight, but I think both sides want to start the process."

That would also mean the gradual decommission of T-64 variants, a mainstay of Ukrainian stocks.

First built in the eastern Ukrainian city of Kharkiv in the 1960s, the T-64 revolutionized tank design by including an autoloader, which cut the crew size from four to three. The aging hulls have been outfitted with upgraded electronics, modern armor and improved gun systems. Modernized variants of the T-64 are the most common tank in the Ukrainian military, according to the London-based International Institute for Strategic Studies.

Many have been damaged by enemy fire, and others are in need of replacement parts after hours of navigating muddy fields and potholed roads. For Ukrainian mechanics, vehicles like it are the most challenging repair, but they are some of the most valuable pieces of equipment on the country's eastern front, where Ukrainian forces have battled Moscow-backed separatists for nearly a decade.

Some T-64 variants have been involved in high-profile moments, such as a lone T-64BV that, according to analysts, ambushed an entire Russian column in an attack captured on video and shared on social media earlier this month. T-64BVs were produced through 1987.

The tanks also underscore Ukraine's long-standing status as a military laboratory.

Ukraine - home to many factories and technical institutions - was a vital part of the Soviet defense industry. By some estimates, as much as 30% of the industry was based there, according to Tracey German, professor of conflict and security at King's College London. Ukraine specialized in missile production and shipbuilding.

The domestically made Neptune missile that Ukrainian forces used to sink the Moskva was based on the design of an old Soviet cruise missile called the Kh-35 that had been produced in Kharkiv - the Detroit of the Eastern European military industry.

After the Soviet Union disintegrated, Ukraine continued to ship helicopter engines, transport aircraft, rockets, missiles and gas turbine engines to Russia, German said.

Ukraine was the world's 14th-largest arms exporter from 2017 to 2021, according to the Stockholm International Peace Research Institute.

The country has upgraded Soviet-designed tanks, vehicles and weapons in recent decades and continued to produce its own arms, including the Neptune missile. The state arms manufacturer, Ukroboronprom, comprises state-owned defense enterprises that employ nearly 70,000 skilled workers.

Russian attacks since February have dealt a blow to Ukrainian weapons production. After Ukraine struck the Moskva, Russia attacked a factory on the edge of Kyiv that had been involved in the production of Neptune missiles and Alder precision-guided rockets.

"Both militaries are using different variations or generations of what is principally Soviet weaponry," said Michael Kofman, research program director in the Russia-studies program at CNA, a nonprofit research and analysis organization.

The bulk of Ukraine's military hardware before the current Russian invasion consisted of late-generation Soviet equipment dating from the 1980s. Ukraine made a push to update some older designs after Russia first invaded the Donbas region in 2014. The Russians, though, had more-sophisticated versions of the same type of weapons, Kofman said.

Both sides are trying to incorporate more modern weapons, including drones, into battlefield arsenals that mainly date from Soviet times, he said. Modern Western antitank guided missiles and manned portable air defense systems have given the Ukrainians a boost.

And the Soviet-era arms keep coming. Eastern European countries have been more than happy to offload their old equipment to Ukraine in exchange for newer models from the West. It's a "win-win-win" situation, Cancian said.

Poland is sending Soviet-designed T-72 tanks to Ukraine, Prime Minister Mateusz Morawiecki said Monday, and Slovakia provided Ukraine with the S-300 antiaircraft missile defense system after Washington offered to replace it with the Patriot system, which is more advanced.

The Czech Republic has sent T-72s, which began service in 1973, to help bolster Ukraine's tank inventory.

Western countries have tried to tread a fine line between arming Ukraine and provoking Moscow. The United States rejected Poland's offer last month to send Soviet-made MiG-29 jets to Ukraine in exchange for American-made planes, out of concern of triggering a direct conflict between Russia and NATO.

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Parker reported from Washington. Bennett reported from eastern Ukraine.