Russia is losing 5 of its soldiers for every Ukrainian it kills in the war's bloodiest battle, NATO official tells CNN

• Russia has lost at least five soldiers for every Ukrainian soldier killed in Bakhmut.
• This is according to an informed estimate by a NATO official who spoke to CNN.
• Ukraine has also suffered significant losses while defending the ruined city, the official said.

The Russian effort to take Bakhmut, a former salt-mining city in eastern Ukraine, has become one of the longest and bloodiest battles in the war. And NATO intelligence estimates suggest it has been especially bloody for the Russian side.

Russian forces have lost at least five soldiers for every Ukrainian soldier killed while defending Bakhmut, a military official with NATO told CNN on Monday. The official, who spoke on the condition of anonymity, said the five-to-one ratio was an informed estimate based on Western intelligence.

The official told CNN that although the ratio was favorable to Ukraine, it had also suffered significant losses.

The unnamed official wasn't the only one to highlight Russian casualties around the city.

Over the weekend, Oleksiy Danilev, the secretary of Ukraine's national security council, claimed an even higher ratio. He said that Russia had lost potentially seven times as many soldiers as Ukraine in Bakhmut, per The New York Times.

However, both sides are known to inflate the other country's casualty figures, and no independent and reliable death toll exists.

Battles around Bakhmut have raged for seven months, with thousands dying and the city now lying in ruin. Trench warfare and the intensity of the fighting have led some to compare the battle to fighting in World War I.

Volodymyr Nazarenko, a senior Ukrainian commander, described the situation in the city as "utter hell" in a Telegram on Sunday, per Reuters.

Initially, Russian forces fighting in Bakhmut were largely drawn from the pro-Kremlin Wagner Group mercenary army, many of them convicted criminals who were used as little more than cannon fodder, The Washington Post reported.

But The Institute for the Study of War (ISW) said in a Monday update that Russian forces have now committed higher-quality special forces operators in a bid to take control of the city.

Over the weekend, Yevgeny Prigozhin, the founder of the Wagner Group, complained about "shell hunger" and a lack of support from the regular army, Insider's Mia Jankowicz reported. He added that Russia's entire front line would collapse if Bakhmut wasn't taken.

But according to the ISW assessment, taking Bakhmut is not "intrinsically significant operationally or strategically" for Russia to win the war, nor would the loss of the city be a strategic setback for Ukraine.

That hasn't stopped both sides from throwing equipment and manpower at it, even as the death toll rises.