Turkish President Recep Tayyip Erdogan voiced opposition on Friday to Finland and Sweden joining NATO, insisting the countries were supportive of Kurdish "terrorist organizations."

While Erdogan did not explicitly commit to blocking the Nordic countries' bid to join the powerful alliance, an objection from Turkey could derail their accession because NATO makes its decisions by consensus. In response, U.S. officials have been scrambling to get clarification from Turkey.

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"We are following the developments regarding Sweden and Finland, but we are not of a favorable opinion," Erdogan said, per the Associated Press. "Scandinavian countries are guesthouses for terrorist organizations. ... They are even members of the parliament in some countries. It is not possible for us to be in favor."

Finland announced its intent to join the alliance on Thursday, and Sweden is expected to do the same within the coming days. Both countries refrained from joining NATO for decades, but Russia's war in Ukraine has seemingly prompted the two nations to rethink their reluctance to join the alliance.

Top of mind for Turkey is the Kurdistan Workers' Party and associated groups. Turkey considers the PKK a domestic terrorist organization and has railed against Western support of Kurdish groups across the region. Support of Kurdish groups in Syria from the United States and other Western nations has been a point of consternation and tension with Turkey.

During his remarks, Erdogan referenced regrets Turkey had over allowing Greece to join NATO's military wing in the 1980s, claiming Greece has shown an "attitude" against Turkey and fomented rifts between other NATO members and Ankara, per Reuters.

The U.S. has been supportive of both Finland and Sweden joining NATO. The State Department has been working to seek clarification on Erdogan's remarks. So far, Turkey is the only NATO member to express opposition to the two nations joining the alliance.

"In terms of the comments President Erdogan has made, we're working to clarify Turkey's position," said Assistant Secretary of State for European and Eurasian Affairs Karen Donfried. "It's not clear to me that Turkey is saying they will oppose."

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Foreign ministers for both Finland and Sweden said they would meet with Turkish officials during a meeting in Berlin to discuss their NATO bid, Axios reported.

Russia has expressed fury over the prospect of Finland and Sweden joining NATO. Finland shares a border with Russia, and Sweden is located close to its western borders. The Kremlin's foreign minister warned Thursday that "Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop the threats to its national security that arise in this regard."