All eyes were on NATO chief Jens Stoltenberg and Ukraine President Volodymyr Zelenskyy at Thursday's European Union summit because their presence will underscore the importance the 27 EU leaders attach to protecting their eastern flank from Russian aggression and beefing up Ukraine's defense capabilities.

Zelenskyy is set to address the gathering by video link and Stoltenberg will host an early lunch at the spring summit for leaders. But the biggest seat at the table will be reserved for something that's not officially on the agenda: the fallout from the stunning weekend mutiny in Russia.

"The mutiny we saw at the weekend demonstrates that there are cracks and divisions within the Russian system. At the same time, it is important to underline that these are internal Russian matters," said Stoltenberg on arrival at the summit.

President Gitanas Nauseda of Lithuania, one of several EU nations bordering Russia, insisted it was all the more reason to take a robust posture toward Russian President Vladimir Putin.

"Some colleagues sometimes say that a strong Putin is less dangerous than a weak Putin. I don't agree with that. We have to move forward and be decisive, because now is a crucial moment of history," he said.

Prime Minister Krisjanis Karins of Latvia, another nation bordering Russia, said that "we cannot control what is happening inside Russia, but we can control what we do on the outside."

Officials from several member states and EU institutions said the chaos and instability created by the rebellion would not only force the EU to double down on its support for Ukraine with commitments for more ammunition but also to ensure fighting and violence does not spill over into the bloc itself.

"There is no room for hesitation," said Estonian Prime Minister Kaja Kallas. "We must continue to increase the price of Russian aggression."