NATO is exploring ways to reinforce security for Finland and Sweden should they ask to join the alliance, even in the period before the other 30 member countries ratify their membership, officials said.

Currently, Sweden and Finland are at their most vulnerable. Since Russia’s invasion of Ukraine in late February, the two Nordic countries have said they are likely to seek NATO membership but have not formally done so. The ratification process takes time, and the countries cannot count on NATO to come to their military aid until it is completed.

NATO Secretary-General Jens Stoltenberg on Thursday said he had discussed with Finland and Sweden ways to make arrangements before they are covered by the group’s security guarantee.

I’m confident that there are ways to bridge that interim period in a way which is good enough and works for both Finland and Sweden,” Stoltenberg said at the European Parliament in Brussels. He did not explain what arrangements were under consideration.

In recent weeks, Sweden and Finland, longtime partners of NATO, have been moving toward requesting formal membership. Only alliance members are covered by the guarantee that an attack on one member is an attack on all, and that the United States and other allies would come to their military aid.

The Pentagon has not told Sweden or Finland that the U.S. would offer a formal security guarantee while their membership is being ratified, officials in Washington said. But the U.S. has bilateral agreements in place with the countries that should help deter Russian aggression aimed at impeding their NATO membership, the officials said.

If Sweden and Finland formally apply, alliance ambassadors are expected to be able to begin the process of ratifying their membership within days, NATO officials said. Russia can be counted upon to oppose their decision and take steps to discourage it.

“When Russia tries in a way to threaten to intimidate Finland and Sweden from not applying, it just demonstrates how Russia is not respecting the basic rights of every nation to choose his own path,” Stoltenberg said Thursday. “So we are in dialogue with Finland and Sweden. And it’s their decision, but if they decide to apply, Finland and Sweden will be warmly welcomed and expect the process to go quickly.”