The front of a plane is mounted on the administrative center of the Motor Sich engine factory in Zaporizhia, Ukraine. The factory employs tens of thousands in Ukraine's fragile east. (Michael Birnbaum/The Washington Post)

Deep into a conflict that has sundered decades-old ties between Ukraine and Russia, Ukraine is still selling military gear over the border to its neighbor, Ukrainian defense industry officials say.

Ukraine’s new leaders have vowed to stop the flow of these defense products, which include key parts for ship engines, advanced targeting technology for tanks and upkeep for Russia’s heaviest nuclear missiles. New laws passed this week bolster their powers to do so. Kiev says helping to arm Russia is tantamount to equipping an enemy during wartime when Moscow is sending support to separatist rebels, a charge the Kremlin has denied.

But Kiev’s pleas for an end to trade ties have run into strong resistance from workers at companies like Motor Sich, here in Ukraine’s industrial heartland, where 27,000 employees build engines tailor-made for Russian military helicopters and planes. Most senior executives here grew up as part of the same Soviet military-industrial club as their Russian peers.

“We have our own party, the party of Motor Sich,’’ company spokesman Anatoliy Malysh said.

The competing pulls are complicating Ukrainian President Petro Poroshenko’s efforts to chart a new course with Moscow at a time when Ukraine and Russia’s economies remain deeply intertwined.

View Graphic Since the breakup of the Soviet Union, Russia has relied on production facilities in south and east Ukraine for critical military supplies.

The increasingly bloody conflict in eastern Ukraine is fraying the nation’s historically close relationship with its far larger neighbor. But after nine months of protest and war, Ukraine’s economy is deep into recession, and it can ill afford to lose jobs, particularly in the key eastern industrial regions that are home to many of the defense plants — and where many people are sympathetic to Moscow.

The close ties between the two nations’ defense industries 23 years after the breakup of the Soviet Union may also be a factor in Russian President Vladimir Putin’s reluctance to allow Ukraine to slip into a Western orbit, analysts say. Ukrainian defense firms have been critical in the Kremlin’s multibillion-dollar efforts to modernize its military, and the conflict between the two countries has set back those projects.

Since the February ouster of Ukraine’s pro-Russian president, Viktor Yanukovych, the Kremlin has unveiled crash plans to make itself less dependent on Ukrainian exports.

Those plans are “key to Russia’s military and economic security, to our technological and production independence, and to our technological sovereignty,” Putin told a meeting of his advisers last month.

Poroshenko in June issued a presidential order to stop the export of defense products to Russia. But it has had little effect on the ground, where factory managers say they have not formally been told to halt their shipments. If they did stop, they say, the region’s economy would grind to a halt. The foot-dragging has frustrated some officials in Kiev.

“How can you work with your enemy? This is gobbledygook,” said a senior Ukrainian security official, speaking anonymously about a sensitive topic.

But, he said, “This decision will mean the death of factories and enterprises.”

The cross-border traffic touches wide portions of Russia’s military capabilities. Ukrainian gears go into the engines of Russian battleships. Ukrainian guidance systems keep Russian early-warning satellites from straying off track. Ukraine-based researchers designed Russia’s heaviest intercontinental ballistic missiles, the SS-18 Satan, and Ukrainian parts go into a wide range of other Russian nuclear weapons.

“It would be unwise to underestimate Russia’s dependence upon the Ukrainian military industrial complex,” said Igor Sutyagin, a London-based military analyst at the Royal United Services Institute, in a research analysis.

Ukrainian research facilities and production plants played key roles in the Soviet Union’s nuclear program at a time when few imagined that Ukraine and Russia would ever become two separate countries. To this day, the hallways of many Ukrainian defense plants are lined with stern-faced portraits of Soviet generals and engineers who took part in developing their country’s fearsome atomic capabilities.

After the fall of the Iron Curtain, many Ukrainian defense institutions maintained their old ties — and their old suspicions of the West.

Ukrainian defense plants do almost a billion dollars of business a year with Russia, said Anton Mikhenko, a defense expert at the Kiev-based Center for Army, Conversion and Disarmament Studies. Ukraine is the world’s eighth-largest defense exporter, according to data from the Stockholm International Peace Research Institute. About 70 percent of Ukraine’s defense exports flow through Russia, analysts say.

Nowhere is there such interdependency as at the Motor Sich factory, which produces nearly all of the engines used in Russian military and transport helicopters, as well as a majority of the country’s transport aircraft engines.

The head of the company’s main Russian competitor, Rostec, told Putin in June that he is able to manufacture 50 helicopter engines a year, although Russia’s demands are for 300 to 350 — most of which are fulfilled by Motor Sich. Rostec pledged to make up the gap by 2016.

In Motor Sich’s hometown of Zaporizhia, a city of 770,000 along the Dnieper River that is less than a three-hour drive from rebel-held territory, little development has taken place since the Soviet era. A 20-foot-tall golden-colored statue of Lenin stands at the end of his namesake boulevard, the city’s main street, and his likeness gestures over the massive hydroelectric dam that helped spark the region’s industrial boom.

Many residents’ lives revolve around the engine factory, which has its own tram system, hospitals and even a system of resorts where its employees can go on vacation, a holdover from the Soviet era.

Nearly all of Motor Sich’s production line flows through Russia. In the company’s headquarters, a map of the world that shows the trail of Motor Sich’s exports revolves around Moscow, not Zaporizhia or Kiev.

“We’re dependent on Russia,” said Malysh, the company spokesman. Leaders in Kiev “think that national interests are more important than the economy. But let them speak to people who live without jobs. We are also patriots,” he said. Motor Sich hasn’t stopped exports to fulfill existing contracts, he said.

Many people tied to the plant say they have conflicted feelings about severing ties with their neighbor.

“Nobody ever thought about this. We’re brothers,” said Alla Kozlovskaya, 47, a teacher at a trade school designed to funnel students onto the factory floor. She said she had family in Moscow.

Other defense firms have already begun to feel the pinch. Yuzhmash, a state-owned rocket factory that supplies and maintains Russia’s heavy SS-18 ICBMs, was so carefully guarded by the Soviet-era Kremlin that its home city of Dnepropetrovsk was sealed to foreign visitors. Now, a senior official at Yuzhmash said, his Russian counterparts are so afraid of traveling to Ukraine that they almost all skipped a major event this year celebrating the firm’s 60th anniversary.

Work has dwindled at the plant because no new Russian contracts have come in, said the official, who spoke on the condition of anonymity because he did not have permission to address the media. The company has slashed pay and cut operations to three days a week, he said.

The disruptions have one regional official warning of the consequences.

“These people might go to Russia, Iran or North Korea,” said Borys Filatov, the deputy governor of the Dnepropetrovsk region. He said that although Ukraine needed to cut ties with Russia, his region’s industries need international support. The fears echoed ones after the collapse of the Soviet Union that highly qualified Soviet nuclear specialists would sell their expertise to rogue states in order to pay the bills.

Russia has in some cases struggled to develop the capacity to manufacture replacements for what it receives from Ukraine. But since the turmoil started, the Kremlin has started multibillion-dollar efforts to attempt to replace what it might lose if the two nations permanently sever ties.

The Kremlin says that it can be self-sufficient within three years, that existing stocks of Ukrainian-made parts can tide over its military in the meantime, and that the project bolsters Russian security even absent the conflict in Ukraine.

The break from Ukraine, Russian officials say, is a patriotic step that should have been taken a long time ago.

“We will be rid of imported component parts and items within the next two or three years, principally those from Ukraine,” Russian Deputy Prime Minister Dmitry Rogozin said this week at a Russian defense trade show, Interfax reported. It is Ukraine that will suffer as a result of the split, he said.

“It scares me even to imagine what will happen to Ukraine,” he said.

Alex Ryabchyn in Zaporizhia and Natasha Abbakumova in Moscow contributed to this report.