She said this during a conversation with journalists before the summit in Brussels, Euronews reports.
"We want to have peace, but we want to have a sustainable peace. And the peace on Russia's terms is not a sustainable. That means that we are going to have a pause of some years, and it's going to go even further because they can," Kallas said.
"(The) Aggressor is provoked by weakness. So that's why I'm also advocating on the European level to do more for defence, increase the defence investment because the aggressor does not take a further step if he sees that we are strong enough and he can't win... That's why we need all of us, all European countries, but also NATO countries to invest in defence," the politician added.
She believes that if Ukraine loses, Russia will take a break for a few years and then spread its aggression to other countries.
"If we really help Ukraine, we don't have to worry about who's next," said the Estonian prime minister.
"If we don't want this war to go any further, we really have to help Ukrainians to defend themselves. It is clearly not only (the) European security architecture that is at stake, but also global security architecture," Kallas added.
She called on the countries of the Ramstein Coalition to allocate 0.25% of their gross domestic product to military assistance to Ukraine. This, in her opinion, "would greatly contribute to Ukraine's victory over Russia."
On March 21, Estonia announced a new €20 million aid package for Ukraine.