President Tayyip Erdogan said on Friday it was not possible for Nato member Turkey to support plans by Sweden and Finland to join the alliance for security considerations.
Ankara's position is crucial, as any enlargement to Nato requires unanimous consent of existing members.
Finland's plan to apply for Nato membership, announced on Thursday, and the expectation that Sweden will follow, would bring about the expansion of the western military alliance that Russian President Vladimir Putin aimed to prevent by launching the Ukraine invasion.
"We are following the developments regarding Sweden and Finland, but we don't hold positive views," Mr Erdogan said. "It is not possible for us to be in favour."
Finland’s leadership has urged politicians to support the country’s bid to join Nato and a final decision by Helsinki is expected to be announced on Sunday.
Sweden appears to be following its neighbour’s lead by edging towards potential Nato membership.
The US was seeking greater clarity on Turkey's position, the top US diplomat for Europe at the State Department said on Friday.
Karen Donfried, assistant secretary for Europe and Eurasian Affairs at the State Department, said the topic will be discussed at the Nato ministerial meeting over the weekend in Berlin as foreign ministers from Turkey, Sweden and Finland among others will be attending.
"In terms of the comments President Erdogan has made, we're working to clarify Turkey's position," Ms Donfried told reporters.
Russia’s ambassador to the EU says European security will be put at risk if Finland and Sweden join Nato.
Vladimir Chizhov said he was “deeply disappointed and saddened” by the moves the two northern European nations are making towards becoming members of the transatlantic military alliance.
During an interview with Sky News, Mr Chizhov was asked if there is a risk of the Ukrainian war spilling over into other European countries.
“Indeed,” he said, immediately pointing to the possible expansion of Nato in north-eastern Europe, saying it “is another element to that”.
He said Finland was “punching above its weight” and risked being turned into a “backwater periphery of Nato”. He suggested the country’s move away from neutrality had damaged “general respect for Finland and its neutral policy”.
The ambassador said Moscow would have to step up surveillance on its 1,340-kilometre border with Finland if it became a Nato member, but ruled out stationing troops and tanks along the boundary.
“It will certainly necessitate [a] certain rethinking of the Russian defence posture,” he said.
“As my government has pointed out, this will necessitate certain military technical measures like improving or raising the degree of defence preparations along the Finnish border.
“You know Finland is not a very big country in terms of population … but it has successfully been pushing above its weight having become in the last few decades a major power in terms of promoting European security architecture,” he said.
Mr Chizhov pointed to the Helsinki Accords, also called the Helsinki Final Act, signed in 1975 primarily to reduce tensions between the Soviet Union and the West by securing their common acceptance of the post-Second World War status quo in Europe.
Finland’s neutrality dates back to the Second World War and in 1948 it signed an agreement with Moscow in which it promised to not join Nato.
During the 1970s Helsinki began to bolster its alliances with the West and went on to join the EU in 1995.
Support for joining Nato has always been low among the Finns, but Russia’s invasion of Ukraine in February prompted a surge in support for joining the alliance.
On Thursday, Finland’s prime minister and president released a joint statement backing the country’s attempt to join the organisation.
A special committee will on Sunday announce Finland's formal decision whether to join Nato.
Nato chief Jens Stoltenberg promises the membership process ― which the US Senate, Germany and France also say they back ― will be "smooth and swift".