Three Russian servicemen were killed Monday after a Ukrainian drone was shot down by air defenses as it approached a military airfield in Saratov Oblast, deep inside Russian territory, according to Russian state news agencies, citing the defense ministry.

The incident took place in the western port city of Engels, some 500 miles (more than 800 kilometers) southeast of Moscow, located on the Volga River. It is the second such attempted attack on the city, which houses the Engels-2 military airfield, a strategic bomber airbase, this month.

Law enforcement agencies are now investigating the incident at the airfield, said Saratov Oblast Governor Roman Busargin on Monday. The comments, posted on his official Telegram channel, came after reports circulated of an explosion in the city.

He added that there were "no emergencies in the residential areas of the city," and that no civilian infrastructure had been damaged. He also extended his condolences to the families of the servicemen, saying the government would provide them with assistance.

CNN has been unable to independently verify Russian reports the drone was deliberately shot down and did not reach its intended target.

In comments Monday, Ukrainian Air Force spokesperson Yurii Ihnat did not claim direct responsibility for the drone, but did suggest the attack was the "consequence of what Russia is doing."

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"If the Russians thought that the war would not affect anyone in the deep rear (of Russia) or anywhere else, they were deeply mistaken. Therefore, as we see, such things are happening more and more often, and let's hope that this will only benefit Ukraine," said Ihnat.

Also on Monday, a spokesperson for South of Ukraine's Security and Defense Forces warned of a possible retaliatory Russian strike, referencing a similar incident earlier this month in the same region.

"This reminds of the events of December 5, so there may be some deja vu, some repetition of this situation, after which [the Russians] launched a massive missile strike," the spokesperson said. "Therefore, we should be prepared for this, take it into account in our plans and do not forget to proceed to the shelter."

Earlier this month, CCTV footage appeared to show an explosion lighting up the sky in Engels. At the time, Gov. Busargin also reassured residents that no civilian infrastructure was damaged and that "information about incidents at military facilities is being checked by law enforcement agencies."

He had acknowledged that information about "a loud bang and a burst in Engels in the early morning" was spreading on social networks and the media.

The Engels-2 airfield is nearly 6 kilometers from where the CCTV footage was recorded in early December.