NATO Secretary General Jens Stoltenberg has said that the issue of Ukraine's membership in the Alliance continues between the Allies and today no one can tell for sure what the outcome of the summit in Vilnius will be like.

He said this at the Brussels Forum organized by the German Marshall Fund of the United States on Wednesday.

"On that issue [of Ukraine's membership in NATO] there are different view in the Alliance. Of course, the only way to make decisions in NATO is by consensus. There are consultations going on now. I will have some phone calls today also on […] how to address the Ukrainians' ambitions for NATO membership. No one is able to tell you exactly what will be the final decision at the Vilnius summit on this issue. […] Having said that I would like to also highlight the following: we also agree on a lot concerning Ukraine and membership. We all agree that Ukraine will become a member of the Alliance. That was actually stated very clearly at the Madrid summit last year and has been repeated many times since we made the first decision in 2008. We all agree that NATO's door is open for new members and that it is for the NATO Allies and Ukraine to decide when they should join, not Moscow – they don't have a veto," he said.

Stoltenberg also recalled that the Allies agree that the most urgent and important task now is to ensure that Ukraine prevails as a sovereign independent nation.

"Because if Ukraine does not prevail, then there is no membership issue to discuss," he said.

Asked whether he thinks this war makes it easier for Ukraine to get at NATO, the Secretary General said, "Yes and no. I think that everyone realizes that to become a member in the mid of war is not on the agenda. And that's not the issue. The issue is what happens when the war ends."

He also said that the war ensures that Ukraine is becoming even closer to NATO.

"One of the things I actually believe and I think that we will agree in Vilnius is the multi-year program by NATO and NATO Allies to help Ukraine the transition from Soviet-era doctrines, equipment and standards to NATO doctrines, equipment and standards. This sounds a bit technical but this is extremely important. […] It's happening but it's a huge task. And by agreeing this program we will then be more ambitious, more concrete on what we are going to do… The aim is to make them fully interoperable with NATO forces regardless of when membership will happen. This is good. It will make them more able to protect themselves and us more able to work with them," Stoltenberg said.

Speaking about security guarantees for Ukraine, he said that the ultimate security guarantee will be NATO membership.

"But as I've just described it is not something that will happen in amidst of the war. What will do is we will approve this program of making Ukraine even closer to NATO and more interoperable with NATO. […] No one can tell exactly when and how this war ends. But what we do know is that when it ends it is extremely important to make sure that it doesn't start again, that Russian aggressive behavior doe not continue and that President Putin does not continue to chip away at the European security – that he just pauses and then starts again. So, we need to make sure that we have arrangements in place to deter further Russian aggression," Stoltenberg said.