Explainer-What you need to know about Finland and Sweden's path to NATO membership

By Anne Kauranen and Johan Ahlander

HELSINKI/STOCKHOLM (Reuters) - Since Russia's invasion of Ukraine, Finland and Sweden have been considering applying for membership of the NATO military alliance, which would mark a major policy shift for the Nordic region.

Here is the latest about the process and the key points under discussion:

WHAT ARE THE KEY STEPS AHEAD?

Finnish President Sauli Niinisto and Prime Minister Sanna Marin said on Thursday that Finland must apply to join the NATO military alliance "without delay".

"We hope that the national steps still needed to make this decision will be taken rapidly within the next few days," they said in a joint statement.

The government and parliament are expected to give their approvals for the decision shortly.

In Sweden, parliament is conducting a security policy review, including the pros and cons of joining the alliance, with the results due on Friday. There is already a majority in parliament in support of NATO membership.

In parallel, the ruling Social Democrats, the biggest party in every election for the past 100 years, will decide on Sunday whether to drop long-standing opposition to NATO membership.

If Finland applies, Sweden is likely to do the same, as it would not want to be the sole Nordic outsider. Other Nordic countries - Norway, Denmark and Iceland - joined the pact as founding members.

Several recent polls suggest a majority of Swedes in favour - something never seen before Russia's invasion.

ANY STICKING POINTS?

Finland and Sweden would like to have some guarantees that NATO member nations would defend them while any application is processed and until they became full members.

Ratification can take a year, NATO diplomats say, as parliaments of all 30 NATO countries need to approve new members.

NATO Secretary General Jens Stoltenberg has said the countries could join "quickly" and that he was sure arrangements could be found for the interim period.

Sweden and Finland have received assurances from the United States, Germany and Britain of support should they come under attack.

Finnish Foreign Minister Pekka Haavisto acknowledged filing a membership application by itself would not bring the two countries under the umbrella of NATO's Article 5, which guarantees that an attack on one ally is an attack on all.

"But at the same time NATO member countries have an interest in that no security breaches would take place during the application period," Haavisto said. Finland could, for instance, hold enhanced military exercises with NATO members during that time.

WHAT IS RUSSIA SAYING?

Moscow has repeatedly warned of "serious consequences" if Finland and Sweden join NATO, saying it would have to strengthen its land, naval and air forces in the Baltic Sea, and raised the possibility of deploying nuclear weapons in the area.

Russia and Finland share a 1,300-km (810-mile) border. The Kola Peninsula, in Arctic northwest Russia pointing eastward from the border with Finland and Norway, is a "strategic bastion" Moscow considers key for its national security, and is also the home of the Russian Northern Fleet.

Russia's second biggest city, St. Petersburg, lies some 170 kms from the border with Finland.

(Additional reporting by Robin Emmott in Brussels, Editing by Angus MacSwan and Emelia Sithole-Matarise)