In a joint press conference in Estonia, when British Prime Minister Boris Johnson and NATO Secretary-General Jens Stoltenberg were asked why there isn't a no-fly zone over Ukraine, they said NATO will provide defensive military support by providing arms, but ultimately NATO is a defensive alliance and they won't fight Russian forces in Ukraine.

"When it comes to a no-fly zone... we have to accept the reality that involves shooting down Russian planes," Johnson said. "That's a very, very big step that is simply not on the agenda of any NATO country."

Estonia's Prime Minister Kaja Kallas said NATO should be prepared to defend the most vulnerable part of NATO, which is the Baltic countries, and need to move from a "forward presence to forward defense, and from air policing to air defense."

Johnson also said, "If Vladimir Putin thinks he's going to push NATO back by what he's doing, he's gravely mistaken. This will end up with a fortified and strengthened NATO on his Western flank -- you'll have more NATO, not less NATO."

-ABC News' Christine Theodorou

A spokesperson for the U.N. High Commissioner for Human Rights said 136 civilians have been killed in Ukraine, including 13 children.

Another 400 Ukrainian civilians have been wounded, spokesperson Liz Throssell said.

Russia's Foreign Ministry claimed U.S. military intelligence launched a massive campaign to recruit contractors from private military companies to send to Ukraine.

"They recruit primarily those employed by Academi, CUBIC and DynCorp [International]," Russian Defense Ministry spokesperson Igor Konashenkov said in a press briefing on Thursday.

Foreign mercenaries who have arrived in Ukraine are performing acts of sabotage and attacking Russian convoys of motor vehicles and aviation, Konashenkov said.

"Foreign mercenaries who earlier arrived in Ukraine are carrying out acts of sabotage and raids on Russian convoys of [military] vehicles and materiel supplies, and on the aviation supporting them," Konashenkov said.

Konashenkov claimed Britain, Denmark, Latvia, Poland and Croatia have legally permitted their citizens to take part in Ukraine hostilities and that the command of the French Foreign Legion plans to send its ethnic Ukrainian troops to aid the Kyiv regime.

Mercenaries from other countries in Ukraine will not receive prisoner-of-war status and will be subject to criminal liability, Konashenkov said.

"I want to officially emphasize that all mercenaries sent by the West to help the Kyiv nationalist regime are not combatants under international humanitarian law. They are not entitled to POW status," Konashenkov said.

Russia said it hopes its current talks with Ukraine on Belarusian soil will soon lead to a peaceful settlement, Russian Foreign Ministry spokesperson Maria Zakharova said.

"We hope the [talks] will lead to a swift conclusion of this situation, the restoration of peace in Donbas, and the return of all peoples of Ukraine to a peaceful and equitable life," she told a briefing in Moscow on Thursday.

- ABC News' Tanya Stukalova