Sweden applied last year to join the defence alliance but Turkey and Hungary have yet to approve the bid.
"We are completely ready and we are waiting for the ratification process to start in Ankara," Billstrom told a news conference in Riga after a meetings of Nordic and Baltic foreign ministers.
Stockholm hopes Turkey will ratify the membership when the Turkish parliament reconvenes in October, as agreed with President Tayyip Erdogan at a summit of the alliance in July.
(Reporting by Anna Ringstrom and Essi Lehto, editing by Terje Solsvik)
