- The US said it is was preparing for threats again Finland and Sweden as they seek to join NATO.
- National Security Adviser Jake Sullivan said the US wouldn't tolerate "aggression" against them.
- Nations don't get formal protection from NATO until they are members, leaving them vulnerable.
- For more stories go to www.BusinessInsider.co.za.
The US said it would not tolerate aggression towards Sweden and Finland as they move through the process of applying to NATO, which could prompt retaliation from Russia.
US National Security Adviser Jake Sullivan said at a White House press briefing on Wednesday that "the United States is prepared to send a very clear message, as are all of our European allies, that we will not tolerate any aggression against Finland or Sweden during this process."
He said that the US defense secretary would coordinate "practical measures" with his counterparts in Finland and Sweden to prepare.
Sullivan noted that Article 5 of NATO's treaty — which obliges all NATO members to respond if any member state is attacked — does not apply until countries are members.
Finland and Sweden formally applied to become NATO members on Wednesday.
US President Joe Biden welcomed the countries' application in a statement on Wednesday, but he hinted that the countries could face additional threats as they wait to see if they become members.
He said that the two nations and the US would "remain vigilant against any threats to our shared security."
Russia has made repeated threats against Sweden and Finland over NATO membership, saying that it would have to retaliate against their efforts to join.
But it has started using softer language in recent days as it became clear that they had decided to join. This week, Putin said he did not consider them joining to be a threat to Russia.
Sweden's prime minister, Magdalena Andersson, said Monday that her country would be in a "vulnerable position" during its application period.
The process for the two countries to become members could now take months, particularly in light of opposition from Turkey, which as a NATO state has the power to veto new members.
As Insider's John Haltiwanger reported, Erdogan may be looking for concessions in response for permitting the new members to join.