Madrid: The North Atlantic Treaty Organization (NATO) has formally invited Sweden and Finland to join the military alliance, the NATO Heads of State and Government said in a declaration.

"We reaffirm our commitment to NATO's Open Door Policy. Today, we have decided to invite Finland and Sweden to become members of NATO, and agreed to sign the Accession Protocols," the statement said on Wednesday.

"The accession of Finland and Sweden will make them safer, NATO stronger, and the Euro-Atlantic area more secure. The security of Finland and Sweden is of direct importance to the Alliance, including during the accession process," the statement added.

NATO Secretary-General Jens Stoltenberg today said that following the invitation, the ratification process of the two countries may take some time. "...we need a ratification process in 30 parliaments; that always takes some time, but I expect also that to go rather quickly because allies are ready to try to make that ratification process happen as quickly as possible."

NATO leaders also approved a new strategic concept that would guide the alliance's work until 2030.

"We have endorsed a new Strategic Concept. It describes the security environment facing the Alliance, reaffirms our values, and spells out NATO's key purpose and the greatest responsibility of ensuring our collective defence based on a 360-degree approach," a summit declaration read.

The concept also sets out the alliance's three core tasks, including deterrence and defence, crisis prevention and management, and cooperative security, according to the declaration.

US President Joe Biden earlier today announced significant US reinforcements of NATO forces in Europe to meet threats across every domain -- land, air and sea, as the much-awaited transatlantic summit began in Madrid amid the raging conflict in Ukraine.

Addressing alongside the NATO secretary-general Jens Stoltenberg, Biden declared new troop movements, equipment shipments and military installations amid the Russia-Ukraine war.

"The United States and our allies are stepping up and proving that NATO is needed now more than ever, and is as important as it ever has been," Biden was quoted as saying by CNN.

Biden said the US plans to establish a permanent headquarters for the Fifth Army Corps in Poland and maintain an additional rotational brigade of 3,000 troops in Romania.

Besides this, Washington will also enhance rotational deployments to the Baltic States, send additional F-35 fighter jet squadrons to the UK and station additional air defence and other capabilities in Germany and Italy.