Finland and Sweden could simultaneously apply to join NATO as soon as mid-May, reports said on Monday, citing sources close to the matter.

Despite tightening cooperation with the military alliance since Russia annexed Crimea in 2014, the Nordic countries opted to stay out.

But Russia's invasion of Ukraine, which it calls a "special operation," has forced Sweden and Finland to examine whether their longstanding military neutrality is still the best means of ensuring national security.

The two countries will together express their wish to join NATO next month, tabloid newspapers Iltalehti in Finland and Expressen in Sweden reported on Monday.

According to Iltalehti, the leaders of Finland and Sweden plan to meet in the week of May 16 and later publicly announce their plans to apply to join the alliance.

Finnish Foreign Minister Pekka Haavisto declined to comment but repeated his longstanding view that he would prefer Finland and Sweden made similar choices.

Swedish daily Aftonbladet reported separately, citing sources close to Swedish government offices, that the United States and Britain had promised Sweden increased military presence, more in-depth military exercises and strong political support from NATO countries during a possible application process.

The Swedish foreign ministry declined to comment on Expressen's and Aftonbladet's reports.

Finland's Prime Minister Sanna Marin said two weeks ago, while visiting her Swedish counterpart Magdalena Andersson that she expected Finland to make its decision on whether to apply for NATO membership within weeks.

Stockholm is conducting a review of security policy, which includes a view on possible NATO membership, with the results due by mid-May.

Separately, Sweden's ruling Social Democrats are also reviewing their long-held objection to NATO membership. That is expected at the latest by May 24.