Turkish President Recep Tayyip Erdogan has doubled down on his opposition to Sweden and Finland joining the NATO alliance, a move that would be historic for the two Nordic countries in the wake of Russia's invasion of Ukraine.

"We will not say 'yes' to those [countries] who apply sanctions to Turkey to join security organization NATO," Erdogan said at a news conference late Monday.

Sweden and Finland have provided refuge to members of Kurdish militant separatist organization the PKK, which Turkey classifies as a terrorist organization and which has carried out attacks in Turkey.

Sweden and Finland are on the brink of applying for membership in NATO, after the governments of both countries expressed their support for the move to abandon their traditional positions of nonalignment between the alliance and Russia.

Sweden and Finland are members of the EU, but not NATO, and the latter shares an 830-mile border with Russia.

For both Sweden and Finland, the decision to apply for NATO membership is monumental and was triggered by Russia's brutal invasion of its neighbor Ukraine, which itself had aspirations to join NATO.

While Turkey has supported Ukraine by sending it weapons, in particular its lethal Bayraktar drones, and has tried to mediate between Moscow and Kyiv, it has so far refused to join its NATO allies in sanctioning Russia.

These might include the easing of Stockholm's bilateral arms embargo on Turkey and some recognition of PKK as a terrorist organization to curtail its fund-raising and recruitment activities," the note said.

But we are basically raising this issue as a matter of national security for Turkey," concerning NATO accession for Sweden and Finland.