Sweden and Finland have submitted their applications to join the North Atlantic Treaty Organisation (NATO).
Their ambassadors made the presentation to Secretary-General Jens Stoltenberg, Dpa reports.
On Wednesday, Stoltenberg described the move as a “historic step” in a “critical moment” for Europe.
The official noted that it came amid Russia’s war in Ukraine now almost three months.
He said the organization would assess the applications and consider the security interests of all allies.
Russian President Vladimir Putin and his Turkish counterpart Recep Erdogan are opposed to the admission.
Erdogan is furious because Sweden and Finland earlier sanctioned his country.
The Turkish leader accused them of supporting “terrorist organisations”.
The comment referred to the banned Kurdistan Workers’ Party (PKK) and the Kurdish militia People’s Defence Units (YPG) in Syria.