By Simon Johnson and Anne Kauranen
STOCKHOLM (Reuters) -Finland and Sweden voiced optimism on Tuesday that common ground can be found with Turkey over its objections to them joining NATO amid a flurry of diplomatic activity aimed at smoothing their path into the 30-nation alliance.
Turkey surprised many NATO allies on Monday by saying it would not support membership for Sweden and Finland after the two countries took the widely anticipated step of agreeing to apply to join the U.S.-led alliance this week.
"Statements from Turkey have very quickly changed and become harder during the last few days," Finnish President Sauli Niinisto said during an address to Sweden's parliament.
"But I am sure that, with the help of constructive discussions, we will solve the situation."
Niinisto said he talked by telephone with Turkish President Tayyip Erdogan a month ago and that the message then had been supportive of Finnish and Swedish membership in NATO.
"But in the last week he has said 'not favourable'," Niinisto said. "That means we have to continue our discussions. I am optimistic."
Niinisto and Swedish Prime Minister Magdalena Andersson will travel to the United States to meet U.S. President Joe Biden on Thursday to discuss the applications, the three administrations said separately on Tuesday.
Finland's Niinisto said quick ratification by the United States could smooth the path toward membership for the two Nordic neighbours, who joined the European Union together in 1995.
"If you have a quick process there, it helps the whole process and the timetable for the whole process," Niinisto told a news conference with Andersson in Stockholm.
Both countries are due to submit their formal applications on Wednesday.
Meanwhile, the White House said the administration was confident NATO can reach consensus about the bids.
"We know there's a lot of support for Sweden and Finland to join NATO," White House press secretary Karine Jean-Pierre said.
Turkey says Sweden and Finland harbour individuals linked to groups it deems terrorists, namely the Kurdistan Workers Party (PKK) militant group and followers of Fethullah Gulen, whom Ankara accuses of orchestrating a 2016 coup attempt.
Swedish Foreign Minister Ann Linde said on Saturday, ahead of talks with her Turkish counterpart at a NATO meeting in Berlin, that Sweden, just like the rest of the European Union, considered PKK a terrorist organisation.
Speaking at a news conference on Monday, Erdogan also said Turkey would oppose the NATO bids from those who imposed sanctions on it. Sweden and Finland laid arms export embargoes on Turkey after its Syria incursion in 2019.
From Stockholm, Andersson said Sweden stood ready to iron out any obstacles in talks with Ankara.
"We are looking forward to having a bilateral dialogue with Turkey," Andersson said.
"I see, in addition to that, when both Sweden and Turkey are members of NATO, there are also opportunities to develop our bilateral relations - between our countries."
The Finnish parliament on Tuesday as expected approved a proposal to apply for NATO membership. Niinisto and the government decided officially on Sunday that Finland would apply but the decision was pending parliament's formal approval.
Finnish Foreign Minister Pekka Haavisto told reporters the country's ambassador to NATO would deliver its application to NATO in Brussels possibly as soon as Wednesday together with Sweden.
How big an obstacle Turkey's objections will be remains to be seen.
"They know that Sweden and Finland inside the alliance is good for the alliance as a whole and I do not foresee they will block this in the end," Anna Wieslander, of the Atlantic Council security policy think tank, said.
"But they will negotiate along the way."
(Reporting by Simon Johnson in Stockholm and Anne Kauranen in Helsinki, additional reporting by Niklas Pollard and Anna Ringstrom in Stockolm, Susan Heavey in Washington, and Jeff Mason and Alexandra Alper in Washington, editing by Niklas Pollard and Nick Macfie)