Why is it so hard to get accurate death tolls in the Russia-Ukraine war?

According to a Monday update from the United Nations’ human rights office, 636 civilians are confirmed to have been killed in the invasion. However, the agency warned, “the actual figures are considerably higher.”

Meanwhile, Russia said on March 2 that it had lost 498 soldiers in Ukraine, while Ukraine’s Defense Ministry said on the same day that more than 5,840 Russian soldiers had been killed. Four days later, U.S. intelligence officials said the Russian death toll was probably between 2,000 and 4,000. (The Ukrainian side currently puts the figure at over 12,000, while Russia has yet to provide a more recent update on its own death toll.)

It’s clearly not easy figuring out what the real death tolls in Ukraine are, and there are different reasons for this, depending on whether you are trying to look at civilian or combatant deaths.

Security situation

When it comes to civilian casualties—which is what the UN human rights office monitors—the biggest issue is the security situation on the ground since Russia’s invasion in late February.

“We have been monitoring civilian casualties since 2014 through our Human Rights Monitoring Mission in Ukraine (HRMMU),” a spokesperson said in an emailed statement. “Clearly since 24 February, our colleagues in the HRMMU have faced a very challenging situation, in some cases having to relocate on a number of occasions given the security situation. They are unable to visit places of incidents and interview victims and witnesses, and have therefore had to adjust their methodology. This means using all other sources of information extensively used, including contacts and partners in places where civilian casualties have occurred.

“Our colleagues are continuing to monitor, corroborate, and report on civilian casualties as best they are able,” the spokesperson said. “Given the security situation and challenges noted above, the real civilian casualty figures are likely to be much higher.”

But civilian deaths are far easier to corroborate than those of combatants, said Shawn Davies, a researcher at the department of peace and conflict at Uppsala University in Sweden, which maintains a massive data set on armed conflicts and their casualties.

Misinformation

“You can generally confirm it because there are local sources on the ground,” Davies told Fortune on Monday, citing Ukrainian municipal and medical organizations as examples. “But combatant deaths are particularly vulnerable to misinformation.”

According to Davies, Ukraine’s government is engaged in a misinformation campaign that aims to boost morale, and Western media is generally happy to accept its claims. That makes it likely that Ukraine’s claims about Russian deaths are exaggerated to some degree. (Ukraine tends to be quieter about its own military death toll, although lawmaker Lesia Vasylenko said Sunday that the figure stood at 1,300.)

Conversely, he added, Russia was “probably” downplaying its own casualties.

In an attempt to get at the truth, Davies’s team is now combing through a “Find Your Own” service that Ukraine’s Internal Affairs Ministry recently set up, replete with videos and pictures, to help Russian families identify dead and captured soldiers.

Ukraine’s authorities have an interest in properly maintaining this service “because this is their way of communicating to the Russian population that they should have an interest in ending the conflict,” Davies said. However, he added, it may be a mistake to assume Ukraine will issue more accurate claims about Russian losses because those figures may end up being cross-checked against Find Your Own.

“The high numbers are probably meant as a morale booster at the moment,” Davies said. “As the existence of the Ukrainian state is threatened in the short term, they have an interest in preserving that by any means.”