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.

While Poland has been welcoming refugees fleeing the destruction in Ukraine, the country will eventually meet its limit on how many people it can take in, Konrad Fijolek, president of Rzeszow, Poland, a city about 60 miles from the Ukrainian border, told ABC News.

Like many cities that border Ukraine, Rzeszow, a town of about 180,000 people, has become a pathway to safety and a lifeline for millions of refugees flooding across the border from Ukraine.

Poland will be able to receive about 1.5 million people, but any more will put the country under strain, Fijolek said.

"If a million or more come, we won't be able to cope," he said.

Some estimates suggest as many as 4 million people could leave Ukraine due to the conflict, but most of the major cities in the country are already full — and officials are attempting to move refugees into smaller cities, a feat that is "not easy," Fijolek said.

So far, all the shelters set up in Poland are temporary, he added.

"We would like to avoid the view of refugee camps here," he said. "We would like people who are escaping from the war to think that what awaits here is proper camp not camps."

In addition, the mental state among the people in Poland is continuing to deteriorate, Fijolek said.

"We as local leaders would like to send the message that if there's any possible action to stop the war [find it] and then try to negotiate, because every day at the border we can see the human tragedy," he said.

-ABC News' Chris Donato and Marcus Moore