British Prime Minister Boris Johnson said Tuesday that talks over Sweden and Finland's membership in NATO with Turkey will be "difficult" but said "progress" had been made.

"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.19:21

This comes after Finland and Sweden's leaders met Turkish President Recep Tayyip Erdogan on Tuesday ahead of a NATO summit in Madrid in an attempt to persuade him to drop his opposition to their membership.

Erdogan has steadfastly refused to approve the Nordic pair's applications, which were submitted in response to the war in Ukraine, despite calls from his NATO allies to do so.

He was scheduled to meet with US President Joe Biden on the sidelines of a meeting focused on the war in Ukraine on Wednesday.

Turkey can effectively block Finland and Sweden from joining NATO because all members must agree to accept new members.

Ankara has accused Finland, and particularly Sweden, of providing a safe haven to Kurdish militants who have been fighting the Turkish state for decades.

Read next: Sweden hints at progress with Turkey, NATO talks

The Turkish president has also urged the two countries to lift arms embargoes imposed on Turkey in 2019 in response to Ankara's military offensive in Syria.

Sweden and Finland went into the NATO meeting expecting Turkey to withdraw its objections after the summit concluded on Thursday.

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"We have made progress. That is definitely the case," said Swedish Foreign Minister Ann Linde. "We are prepared for something positive to happen today, but also for it to take more time," she added. "We must be patient and continue discussions even after the summit."

Read next: Swedish PM hoping to resolve Ankara dispute before NATO summit

Finnish President Sauli Niinisto said he was neither "optimistic nor pessimistic at this stage." Erdogan, however, said he wanted to see the outcome of Monday's preparatory talks in Brussels before deciding whether Sweden and Finland had done enough to overcome his objections to their NATO membership.

"We will see what point they (Finland and Sweden) have reached," he said on Monday before flying to Madrid for the summit.  "We do not want empty words. We want results."

'Interest of the alliance´

Speaking to reporters aboard Air Force One, US National Security Advisor Jake Sullivan stated that Biden and Erdogan would meet "at some point" on the sidelines of the summit on Wednesday.

However, he stressed that the US would not play a "brokering role" and would leave the NATO secretary-general in charge.

"Rather, we're going to do what many other allies have done which is indicate publicly and privately that we believe it is in the interest of the alliance to get this done," he added.

Read next: Biden may mediate Turkey's talks with Finland, Sweden: Reports

"And we also believe that Finland and Sweden have taken significant steps forward in terms of addressing Turkey's concerns."

Analysts believe Erdogan and Biden's meeting will be critical in breaking down Turkey's resistance to Sweden and Finland's bids to join the Western defense alliance in response to the war.

Because of US concerns about "human rights under Erdogan," the two leaders have had a chilly relationship since Biden's election. They both last met briefly in October on the sidelines of a G20 summit in Rome.

Despite #Turkey's refusal and #Russia's warning, #Finland and #Sweden are still adamant about joining the alliance, sparking fears for the future of #Europe.#NATO pic.twitter.com/AvuwyoU2Kg

— Al Mayadeen English (@MayadeenEnglish) May 18, 2022