June 16 (UPI) -- As Russia kept up attacks in eastern Ukraine on Thursday, a Ukrainian official said that "tens of thousands" of people have been killed since the start of the war in February.
Ukrainian Defense Minister Oleksiy Reznikov specified the death toll in an interview, saying the number of dead is well above all "official" counts that have been reported so far. The United Nations has said in periodic updates that a few thousand have been killed.
However, getting an accurate count of the dead in Ukraine is a difficult task considering the ongoing fighting in many regions of the country, where it's not safe for officials.
Reznikov told CNN that the death toll is surely in the tens of thousands and that he hopes it's under 100,000.
On Wednesday, the United Nations High Commissioner for Human Rights put the figure at roughly 10,000, including about 4,500 Ukrainian civilians and 200 children.
"Most of the civilian casualties recorded were caused by the use of explosive weapons with a wide impact area, including shelling from heavy artillery and multiple launch rocket systems, and missile and airstrikes," the U.N. report said.
Meanwhile, the families of two American veterans who have been fighting with Ukrainian forces fear that both have been captured by Russian troops.
The families say that the men -- 39-year-old Alexander John-Robert Drueke and 27-year-old Andy Tai Ngoc Huynh, both from Alabama -- have been fighting under the command of Ukraine's 92nd mechanized brigade. They went missing on June 9 near the town of Izbytske, which is located in northeastern Ukraine about 30 miles from Kharkiv.
State Department spokesman Ned Price said that the government is aware of the men's reported capture in a briefing Thursday.
"We are closely monitoring the situation. We are in contact with Ukrainian authorities, as well as with the International Committee of the Red Cross and the families of the two reported missing U.S. citizens. Of course, we're not able to offer any more on that front because of privacy considerations" Price said.
Price added the State Department was also aware of a third American who had been reported missing while voluntarily traveling to Ukraine to join the war effort.
The third missing man was Grady Kurpasi, a former U.S. Marine, his wife, Heeson Kim, told CNN.
George Heath, a friend of Kurpasi's, told CNN that Kim and other close friends last heard from Kurpasi between April 23 and 24.
Heath said Kurpasi served in the Marines for 20 years and traveled to Ukraine to volunteer alongside Ukrainians but initially did not envision himself fighting in the war
"For him personally, he has a skillset that he feels he can give back," he said. "He wanted to go and help the Ukrainian people. He wasn't really planning on fighting."
Price on Thursday urged American citizens not to travel to Ukraine, citing "the attendant danger that is posed by Russia's ongoing aggression."
Russia launched its invasion on Feb. 24 and has been focused for weeks on gaining control of eastern Ukraine, which is home to the Donbas region. For nearly a decade, pro-Russia separatists there have fought with the Ukrainian government over who controls the region.
On Thursday, Russia continued its advance in Severodonetsk in Luhansk oblast and outnumbered Ukrainian troops as they moved to solidify gains. One Ukrainian official estimated this week that Russia controls about 80% of Severodonetsk, which is believed to be the last major city in the east that's still under some Ukrainian control.
U.S. Joint Chiefs of Staff chairman Gen. Mark Milley said that Moscow's troop advantage over Ukraine has played out time and again since the fighting began.
"The math problem is very difficult for the Ukrainians," Milley told NPR, adding that Russia began to find some success when it concentrated more forces at a decisive point.
After failing to capture the capital, Kyiv, Russia refocused by withdrawing some forces and placing a heavier emphasis on the eastern part of the country.
Britain's Defense Ministry said on Thursday that the numbers advantage has definitely helped Russia in its bid to capture Severodonetsk. Early this week, Russian forces isolated the city by destroying three main bridges that led into the area.
"With the bridges highly likely destroyed, Russia will now likely need to either conduct a contested river crossing or advance on its currently stalled flanks to turn tactical gain into an operational advantage," the British ministry, which has been monitoring the war since the beginning, said in a tweet.
"For both sides fighting in contested towns, front line combat is likely increasingly devolving to small groups of troops typically operating on foot. Some of Russia's strengths, such as its advantage in the numbers of tanks, become less relevant in this environment. This is likely contributing to its continued slow rate of advance."
Russia's military moves on Thursday came as some European leaders visited Ukraine and met with Ukrainian President Volodymyr Zelensky. They included French President Emmanuel Macron, German Chancellor Olaf Scholz, Italian Prime Minister Mario Draghi and Romanian President Klaus Iohannis.
Ukraine has said on several occasions since February that it hopes to join the European Union. The 27-member alliance has been receptive to Ukraine's request, but EU officials have said that making the addition would take time.