Turkey has opposed Sweden and Finland's membership of NATO, accusing them of harbouring individuals linked to the Kurdistan Workers Party militant group (PKK) and followers of a cleric Turkey accuses of orchestrating a coup attempt in 2016.
What are these groups?
THE PKK
The Kurds are a minority living in a region straddling the borders of Iraq, Iran, Syria, Turkey and Armenia. In Turkey, they make up about 20% of the population. The PKK was founded in 1978 with the aim of creating an ethnic homeland in southeast Turkey. It took up arms against Turkey in 1984, a conflict in which more than 40,000 people have been killed.
Western governments including the United States and European Union designate the PKK as a terrorist organisation.
The conflict extends into northern Iraq, where Turkey regularly attacks what it says are PKK camps and ammunition stores. It also spread into Syria as that country descended into war from 2011, and a group inspired by the ideology of PKK founder Abdullah Ocalan - the YPG - emerged as an armed faction.
THE YPG
The YPG, or People's Protection Units, established a foothold in northern Syria at the Turkish frontier early in Syria's conflict. The group joined a U.S.-led coalition fighting Islamic State, becoming the spearhead of a wider militia, the Syrian Democratic Forces, which enjoys U.S. support.
Turkey says the YPG and PKK are one and the same, and views their Syrian foothold as a national security threat.
While Turkey brands the YPG as a terrorist group, Western governments do not. U.S. ties to the SDF have annoyed Ankara and been a source of tension for years, with Turkey particularly angered by Washington's supply of weapons to the group.
Turkey has launched several incursions into northern Syria. One such incursion, in 2019, prompted Finland and Sweden to ban some arms sales to Turkey. Lifting this is one of Ankara's demands.
GULENISTS
Apart from the Kurds, Turkey complains about Western countries harbouring supporters of U.S.-based Islamic cleric Fetullah Gulen, whose Hizmet (Service) group once had wide influence in Turkey's military, judiciary and education.
Turkey blames the Gulenists for a failed coup attempt in 2016. Since then, more than 60,000 people in the army, judiciary, civil service and education have been detained, suspended or investigated for alleged links to the group.
WHAT DOES TURKEY WANT?
Turkey says Helsinki and Stockholm failed to agree to Ankara's requests to extradite "dozens of terrorists" over the last five years, including individuals linked to both the PKK and Gulen.
Turkey has said it will not look positively on the Nordic states' NATO membership unless they clearly show cooperation on the fight against terrorism and other issues, and lift the arms embargo.
Erdogan spoke to both countries' leaders on Saturday, telling Swedish Prime Minister Magdalena Andersson that Ankara expected concrete steps to address its concerns, and Finnish President Sauli Niinisto that failing to deal with terrorist organisations posing a threat to a NATO ally would not suit the spirit of alliance.
Many analysts have said Erdogan may be aiming to use this moment to press Washington over some long-standing issues that have weighed on ties, including support for the YPG.
WHAT DO FINLAND AND SWEDEN SAY?
In her call with Erdogan, Andersson said she emphasised that Sweden welcomed the possibility of cooperation in the fight against international terrorism. Sweden supports the fight against terrorism and agrees with the PKK's terrorist listing.
Finland has said it condemns terrorism in all its forms and is open to discussing Turkey's concerns. Niinisto said he held "open and direct" talks with Erdogan and agreed to continue close dialogue.
(Writing by Tom Perry and Ece Tokasbay; Editing by Daren Butler)
