Mr Putin did not make a specific threat of retaliation over Finland’s move, but the Russian foreign ministry has indicated there will be retaliation.
Following the phone call between Mr Niinistö and Mr Putin on Saturday, the Kremlin said the Russian leader had stressed the “end of the traditional policy of military neutrality would be a mistake since there is no threat to Finland’s security”.
It added: “Such a change in the country’s political orientation can have a negative impact on Russian-Finnish relations developed over years in a spirit of good neighbourliness and co-operation between partners.”
Mr Niinistö said he had told Mr Putin how recent moves by Russia, along with the invasion of Ukraine, “have altered the security environment of Finland”.
A possible stumbling block to Swedish and Finnish membership of Nato is Turkey, after President Recep Tayyip Erdogan accused both countries of harbouring “terrorist organisations” and said he did not support their applications.
For decades the PKK has fought with Turkey for a Kurdish homeland.
Turkey also objects to a Syrian Kurdish militia called the People’s Protection Units (YPG) ,which they say is an extension of the PKK – even though they have been allied with the US in the fight against so-called Islamic State.