The two-page letter gives 14 examples of steps taken by Sweden to show it "is fully committed to the implementation" of a memorandum it signed with Turkey and Finland in June, which resulted in NATO member Turkey lifting its veto of their applications to the trans-Atlantic security alliance.

Sweden and Finland launched their bids to join NATO in May in response to Russia's invasion of Ukraine, but ran into objections from Turkey, which accuses the two Nordic countries of harbouring what it says are militants from the banned Kurdistan Workers Party (PKK) and other groups.

Stockholm and Helsinki deny harbouring terrorists but have pledged to cooperate with Ankara to fully address its security concerns, and to lift arms embargoes. Yet Turkey's President Tayyip Erdogan said as recently as Oct. 6 that its demands had not yet been met.

In its letter to Turkey, Sweden said that "concrete action has been taken on all core elements of the trilateral agreement".

Sweden's security and counter-terrorism police, Sapo, "has intensified its work against the PKK", and it made "a high-level visit" to Turkey in September for meetings with Turkey's MIT intelligence agency, the letter said.

Sweden's foreign ministry and the communications arm of Erdogan's office each did not immediately respond to a request for comment on the letter.

Swedish officials delivered the letter, which was not previously reported, to Erdogan's office and the foreign ministry at the weekend, a source familiar with the situation said, requesting anonymity due to sensitivity over it.

The letter was meant to reassure Turkey of Sweden's efforts amid ongoing bilateral talks and to encourage ultimate approval of the NATO membership bid, the source added.

According to the letter, Swedish authorities "carried out new analyses of PKK's role in threats to Sweden's national security and in organised crime (and) this is likely to lead to concrete results."

The PKK, which took up arms against the Turkish state in 1984, is considered a terrorist group by Turkey, the European Union and the United States. As part of talks over the June memorandum, Turkey has sought the extradition of 73 people from Sweden and a dozen others from Finland, where it is concerned with other groups.

The letter says Stockholm extradited one Turkish citizen on Aug. 31 upon Ankara's request, after an Aug. 11 decision, and that a total of four extraditions have been made to Turkey since 2019.

Extraditions were discussed by a Swedish delegation visiting Ankara in early October, according to the letter.

"Sweden is committed to address...pending extradition requests of terror suspects expeditiously and thoroughly," taking into account Turkish intelligence and in accordance with Swedish law and the European Convention on Extradition, the letter said.

Turkey will continue consultations with Sweden and Finland "to pursue full implementation of the memorandum," Turkish diplomatic sources told Reuters. However steps "need to be taken...(in) combatting terrorism, prevention and punishment of incitement to terrorism, improvement of security and judicial cooperation," the sources added.

The parliaments of all 30 North Atlantic Treaty Organization (NATO) member states' must approve Sweden and Finland's bids, which would mark a historic enlargement of the alliance as the war in Ukraine continues.

In a sign that talks were progressing, Sweden's foreign minister said on Friday he expects the last two holdouts, Turkey and Hungary, to vote soon on its NATO applications.

Erdogan was quoted by Turkish broadcasters as saying on Friday that Sweden's newly appointed Prime Minister Ulf Kristersson backs the fight against terrorism and that they would meet to discuss the NATO bid and extraditions.

A day earlier Kristersson said after meeting with NATO's secretary general that his government "will redouble efforts to implement the trilateral memorandum with Finland and Turkey".