"We look forward to soon welcoming Sweden as the 32nd (member of NATO). And to be clear, we look forward to that happening before the (NATO) summit in July," US Secretary of Defence Lloyd Austin said on a visit to Sweden.

"So we encourage our allies, Turkey and Hungary, to ratify Sweden's accession as soon as possible," he said.

Highlighting the US support for Sweden, Austin said the US had increased its military presence in the Scandinavian country.

"To underscore our deep support, the department (of defence) has increased the number of ship visits and bomber task force missions and high level engagements with Sweden," he said.

Ending two centuries of neutrality and military non-alignment, Sweden and neighbouring Finland announced bids to join NATO in May 2022, in the wake of Russia's invasion of Ukraine.

But while Finland managed to become the 31st member of NATO on April 4, Sweden's bid still faces opposition.

New members need to be unanimously ratified by all members of the alliance, and Sweden still faces opposition from Ankara and Budapest.

Sweden has especially angered Turkey by refusing to extradite dozens of suspects that President Recep Tayyip Erdogan links to a failed 2016 coup attempt and a decades-long Kurdish independence struggle.

Hungary meanwhile has "an ample amount of grievances that need to be addressed" before it can ratify the Swedish bid, presidential spokesman Zoltan Kovacs said in March, accusing Swedish representatives of being "repeatedly keen to bash Hungary" on rule-of-law issues.