• Russia has repeatedly stated it's against NATO's enlargement and it has named this as was one of the reasons for its invasion of Ukraine. 
• It is unclear how the Kremlin would react if both Sweden and Finland join the alliance.
• NATO's Secretary General Jens Stoltenberg has previously said both nations would be warmly welcomed.

SALZBURG, Austria — Finland and Sweden need to join the North Atlantic Treaty Organization (NATO) now while Russia's Putin is focused on Ukraine, the alliance's former chief told CNBC.

The two Nordic countries have been considering joining NATO in the wake of Russia's unprovoked invasion of Ukraine. Becoming NATO members would represent a sharp U-turn in their policies towards the Kremlin after years of taking a neutral approach. Finland and Sweden are due to announce their plans in the coming days.

"As far as Finland and Sweden are concerned, I think there's a window of opportunity for [the] two countries to join, exactly now because Putin is preoccupied elsewhere. He can't do anything about it," Anders Rasmussen, former NATO secretary general, told CNBC Saturday.

Russia has repeatedly stated it's against NATO's enlargement and it has named this as was one of the reasons for its invasion of Ukraine. 

In addition, the Kremlin has also said if Stockholm and Helsinki were to join the alliance, then it would have to "rebalance the situation."

It is unclear how the Kremlin would react if both nations move ahead with their memberships.

However, their accession would lead to doubling the current NATO-Russia border and significantly add more military power to the alliance.

NATO's Secretary General Jens Stoltenberg has previously said both nations would be warmly welcomed.

But it could take "some months" before their memberships were to become official, Rasmussen told CNBC.

"Even if it's considered an urgent procedure, and it is, it will take some months because you have to go through 30 Parliaments before it can be ratified all over NATO," he said.

NATO currently has 30 members, including the United States.

"It will take some months and during that period both Finland and Sweden could potentially be exposed to Russian intimidation or even threats, and that's why we have to guarantee their security," Rasmussen said, "as if they were already members of NATO."

These security guarantees would have to come from individual members of NATO as the alliance's famous Article 5 — which states that an attack on one NATO member is an attack against all — would only apply to Finland and Sweden once their applications were ratified by all the 30 NATO members.

Russia's unprovoked invasion of Ukraine has led to a shift in defense policy in Europe. Countries have announced a lot more spending on their military capabilities, have sent weapons to Ukraine and — in the case of Finland and Sweden — it has led to more public support for joining NATO.

"You should also understand the Swedish and the Finnish [potential] decisions was a message that there is no neutral countries on the border of Russia. And this is a new reality, even during the Cold War, it was not like this," Ivan Krastev, a political analyst, told CNBC Friday.

"Before [Russia's invasion of Ukraine] it was not clear what is the difference between member of NATO and just being friends of the United States. Now, it is quite clear that being a member of NATO means Article Five, and being just friends of the United States does not. And this is why Finland and Sweden should move from friends to members," he added.