As Trudeau warns democracy faces a 'defining moment,' NATO reinforces its eastern flank

NATO will deploy four more battle groups in eastern Europe as part of a wide-ranging reinforcement of the region following Russia's invasion of Ukraine, the secretary general of the western military alliance said Wednesday.

The additional troops will be sent to Slovakia, Hungary, Bulgaria and Romania, Jens Stoltenberg told a media availability in Brussels ahead of Thursday's meeting of NATO nation leaders.

His remarks came as Prime Minister Justin Trudeau, delivering an address to the European Parliament, warned that free nations can't afford to take democracy for granted.

He said western nations have watched the storm building for years and now face "a defining moment."

WATCH: Prime Minister Justin Trudeau addresses European parliament

Speaking to packed public galleries, Trudeau received a standing ovation when he said the European Union and NATO are more united than ever in the face of Russian President Vladimir Putin's war. He said the world "cannot let Ukraine down."

"Putin's attack on Ukraine is an attack on the values that form the pillars of all democracies," Trudeau said.

"We have a responsibility to make the case to people about why these values matter so much — not just to Ukrainians but to us all. We must recommit ourselves to the work of strengthening our democracies, and demonstrate the principled leadership people are looking for."

Trudeau is in Europe for a meeting of NATO and G7 leaders who are deciding how the military alliance and the world's leading economic democracies can do more to help Ukraine defend itself.

Senior NATO commanders, speaking on background Wednesday following Stoltenberg's remarks, said they estimate between 7,000 and 15,000 Russian troops have been killed fighting in Ukraine in the month since the invasion began.