President Joe Biden has stressed “close security and defense cooperation” between the United States, Finland, and Sweden in a joint call with those countries’ leaders in which he also encouraged their looming NATO bids amid Russia’s attack on Ukraine.

A White House readout of the call said Biden also “reiterated their shared commitment to continued coordination in support of Ukraine and the Ukrainian people affected by the war” to Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinisto.

The White House and Pentagon later said they were seeking clarification on potential objections by NATO ally Turkey to Finnish and Swedish membership.

The Swedish and Finnish governments this week have laid out plans to commit their countries to applying for NATO membership as soon as this weekend, a result of the threat projected from Moscow.

“President Biden underscored his support for NATO’s Open Door policy and for the right of Finland and Sweden to decide their own future, foreign policy, and security arrangements,” the White House account said of the call.

Many members of the alliance have already expressed support for applications from Sweden and Finland, both of which have traditionally remained neutral.

But Turkish President Recep Tayyip Erdogan on May 13 said he did not have a “positive opinion” of their membership.

Longtime NATO member Turkey has repeatedly criticized Sweden and other Western European states for their handling of groups deemed terrorists by Ankara, including the Kurdish militant groups PKK and YPG, and the followers of U.S.-based Islamic cleric Fethullah Gulen.

“Scandinavian countries are like a guesthouse for terror organizations,” Erdogan said after Friday Prayers in Istanbul.

Erdogan says “Gulenists” carried out a coup attempt in 2016 and his administration has detained tens of thousands over their alleged support or sympathies for the group. Gulen and his supporters deny the accusation.

Erdogan’s opposition could pose a problem for a process otherwise seen as clear sailing, since new NATO members need unanimous agreement.

Hours after Erdogan’s comment, the White House and Pentagon said they were “working to clarify Turkey’s position” regarding Sweden and Finland.

“Nothing changes about their standing in the NATO alliance,” Pentagon spokesman John Kirby said. “We’re working to better clarify [their] position.”

Finland, which shares a 1,340-kilometer border and a turbulent past with Russia, said on May 12 that it must apply to join the NATO military alliance “without delay.”

Sweden’s becoming a NATO member would have a stabilizing effect and would benefit all Baltic sea states, Foreign Minister Ann Linde said on May 13.