Progress on Finland and Sweden's NATO membership bids will only be possible if concrete steps are taken addressing Turkey's national security concerns, President Recep Tayyip Erdogan's spokesman told his Nordic counterparts in calls on Wednesday.

Ibrahim Kalin, who is Erdogan's chief foreign policy adviser, held calls with counterparts from Sweden, Finland, Germany, Britain and the United States to discuss the proposed NATO enlargement, according to a readout from Erdogan's office.

"It was underlined that if Turkey's expectations were not met, the progress of the process would not be possible," it said.