The President of Finland said he trusts his country and Sweden will be admitted into Nato by July.
If the issue drags on, the entire process of admitting new members into the military alliance will become questionable, Sauli Niinisto said in an interview with the Finnish news agency STT.
Lithuania is set to host a Nato summit in the Baltic nation’s capital Vilnius on July 11-12.
Nato requires unanimous approval from its existing members to admit new ones.
Turkey and Hungary are the only nations in the 30-member military alliance that have not endorsed Sweden and Finland’s accession.
While Hungary has pledged to do so in February, Turkey has not indicated any willingness to ratify the two countries’ membership.
“I think that under no circumstances will he allow himself to be influenced by any public pressure,” Mr Niinisto said.
“But if something opens up during the bilateral talks between Turkey and the United States, it might have an impact.”
Turkey has been holding off approving Sweden and Finland’s membership into Nato following a recent series of demonstrations in Stockholm by activists who have burned the Koran outside the Turkish Embassy and hanged an effigy of Mr Erdogan.
In January, Ankara indefinitely postponed a key meeting in Brussels that would have discussed the two Nordic countries’ entry into Nato.
Mr Niinisto said Finland and Sweden heard many encouraging statements from Nato last spring about a smooth and painless progress to membership.
He said that did not happen, adding the delay is not only a problem for Finland and Sweden.
“I can see that this has already become a problem for Nato. Clearly, Nato countries have also been surprised,” Mr Niinisto said.