A high-ranking Russian officer, Col. Ruslan Galitsky, who was accused of commanding pro-Russian separatists in eastern Ukraine, has been killed in Aleppo, Syria. Russian President Vladimir Putin personally confirmed the colonel’s death.

The Russian Ministry of Defense announced on Wednesday that Galitsky had been killed, and Putin subsequently said that Galitsky had suffered fatal wounds when a Russian military field hospital in Aleppo’s al-Furqan neighborhood was shelled on Monday.

A Russian military doctor and a nurse reportedly were killed in the same attack.

According to the state-owned RIA Novosti news agency, Galitsky was acting as a military adviser to the Syrian regime, which has made a rapid three-week advance—with the help of Russian air power and Iranian-sponsored Shia proxy forces—through about 75 percent of East Aleppo. The area has been a stronghold of the Syrian opposition for the last four years.

Vladimir Kuzmin, head of the Saint Petersburg branch of the Officers of Russia veterans’ organization, told the independent Fontanka newspaper that Galitsky had been due to be promoted to the rank of Major-General as soon as December 12.

The death of such a high-ranking Russian military commander in Syria would always garner attention, but Galitsky has a particularly interesting history.

In May this year, the Ukrainian Ministry of Defense publicly accused Galitsky of commanding militants in the war-torn east of that country.

According to Ukrainian military intelligence, Galitsky was the deputy chief of staff at the Center of Territorial Troops in the Rostov city of Novocherkassk, around 50 kilometers from the Ukrainian border.

That Galitsky, who was born in Ukraine, did indeed serve at a military unit based in Novocherkassk is not in doubt. On February 16, he was decorated for his military service by the governor of the Rostov region, Vasily Golubev. At this time, he was named as the “chief of the directorate of operations at military unit 64722 of the Southern Military District.”

Unit 64722 is rather mysterious.

A search of Russian military databases reveals absolutely nothing. That the unit exists is proven by several military court records of disciplinary or pay-related cases lodged by servicemen deployed there.

In May this year, an anonymous user on the Voronezh Q&A website asked if the unit existed, and gave the address as 36 Atamanskaya Street in Novocherkassk.

They received no reply, but the address matches that of a former military academy, officially dissolved in 2011. In December, 2014, the same address was given in replies on a Novocherkassk forum to a plea for help from a mother trying to contact her son. She she had been told he was transferred to another unit, 65246, but had been unable to find any information on the base or establish direct contact by phone.

Reporting last year on a third such ghost unit based in Novocherkassk, Ukrainian investigative bloggers “InformNapalm” suggested that the Russian military was using these non-existent units as fronts for deploying troops into Ukraine.

That Galitsky was decorated on February 16 this year may well be significant. The date marked the anniversary of a major Russian offensive in Ukraine to seize the city of Debaltsevo.

Intriguingly, it was the very tank unit that led this offensive that Galitsky wound up commanding later in the year: the 5th Independent Tank Brigade, based in Ulan-Ude in Russia’s far-eastern Republic of Buryatia, playing a leading role in the Selenga-2016 military exercises held in August and September with the Mongolian armed forces.

The 5th Independent Tank Brigade (OTB) took part in some of the fiercest fighting of the Ukraine war, engaging Ukrainian tanks to encircle Debaltsevo. Dorzhi Batomunkuyev, a tank gunner in the Brigade, suffered horrific burns after his tank was knocked out near the village of Logvinovo and described his deployment in detail to Novaya Gazeta’s Yelena Kostyuchenko.

That the 5th OTB fought in Ukraine has also been demonstrated by Vice News’s Simon Ostrovsky, Bellingcat, and InformNapalm.

Galitsky is not the only member of the 5th OTB to have wound up dying in Syria.

On June 19 of this year, another colonel in the brigade, Vladimir Bekish, was killed by shelling in the Aleppo region. Bekish, a 42-year-old from the Belarusian town of Grodno, was reportedly the head of the intelligence company in the 5th OTB.

So what are commanders from a Russian tank brigade doing dying in Aleppo, where the Russian military operation is supposedly limited to air support and humanitarian activity?

According to Kathleen Weinberger, a Russia analyst at the Washington, D.C.-based Institute for the Study of War, “Russia is using the battlefields in both Ukraine and Syria to test and develop its hybrid war capabilities, particularly its ability to coordinate air power with a ground fight. Aleppo is an ideal test case for the Russians to practice and refine that capability in a dense urban environment.”

Russia has in the past rotated units and officers involved in its undeclared dirty war in East Ukraine into Syria to help steer Assad’s recapture of lost terrain, and coordinate with Iranian reinforcements.

Weinberger told The Daily Beast that Gen. Aleksandr Dvornikov, who led Russia's Syria operation, was rotated back into the Southern Military District, indicating that the Russian military may be applying lessons learned from the prosecution of the one conflict to the prosecution of the other.

Galitsky, Weinberger added, was likely acting in an advisory role embedded with a Syrian armored unit and helping to coordinate the regime’s effort to seize the remainder of East Aleppo.