"Finland and Sweden, breaking decades of historic neutrality, are now wanting to join. It will be a difficult conversation," he told reporters on the plane taking him to Madrid for a NATO summit.

"I think the key thing here is that... progress is being made," he added.

"I think it's very important for our alliance that those two countries should be brought in."

For a new country to join the military alliance, all existing members must agree.

But Turkey has so far refused to greenlight their applications, effectively vetoing Finland and Sweden from joining.

Ankara has accused both nations, particularly Sweden, of offering a safe haven to Kurdish militants who have been waging a decades-long insurgency against the Turkish state.

Turkish President Recep Tayyip Erdogan has also called on the pair to lift arms embargoes imposed on Ankara in 2019 over its military offensive in Syria.

The leaders of Finland and Sweden met Erdogan on Tuesday just hours before the official opening of the Madrid NATO summit in a bid to get him to drop objections to them joining.