NATO Secretary-General Jens Stoltenberg says he has met with Finland’s prime minister and spoken to Turkey’s president as he seeks to overcome Ankara’s opposition to Finland and Sweden joining the alliance.

Stoltenberg, who visited Washington this week, tweeted late on June 3 that he met with Finnish Prime Minister Sanna Marin while he was there and discussed “the need to address Turkey’s concerns and move forward” with the Finnish and Swedish membership applications.

Russia’s invasion of Ukraine has prompted Finland and Sweden to apply to join NATO. But Turkish President Recep Tayyip Erdogan has said he is against the accession of the two Nordic countries because of what he called their support for “terrorist organizations,” a reference to the banned Kurdistan Workers Party (PKK) and the Kurdish militia People’s Defense Units (YPG) in Syria.

In a separate tweet, Stoltenberg said he had a “constructive phone call” with Erdogan. He called Turkey a “valued ally” and praised Turkish efforts to broker a deal to ensure the safe transportation of grain supplies from Ukraine amid global food shortages caused by the war.

Stoltenberg added that he and Erdogan would continue their dialogue.

Senior officials from Sweden, Finland, and Turkey will gather in Brussels next week to discuss Ankara’s opposition to the applications. Membership in the alliance must be unanimously approved by current members.