NATO's flag (Photo: REUTERS)

Speaking in parliament, Swedish Foreign Minister Tobias Billstrom was optimistic about chances to join the alliance by the time of the NATO summit in Vilnius, Lithuania, in July.

Turkish President Tayyip Erdogan last week said Turkey's parliament will start ratifying Finland's accession but held off approving Sweden's bid. Hungary's legislature said it will vote on ratification of Finland's accession next Monday, but not Sweden's.

Sweden and Finland asked to join NATO last May. 28 of 30 members have ratified their accession. Turkey and Hungary have yet to do so, with Turkey being considered the bigger obstacle.