The Syrian Kurdish YPG militia and its political branch PYD are considered by Turkey extensions of the Kurdistan Workers Party (PKK), which launched an insurgency against Turkey in 1980 and is regarded as a terrorist group by Turkey, the United States and the European Union.

Sweden, along with the United States and several other NATO countries, has supported the YPG in the fight against Islamic State.

However, Turkey has vowed to block Sweden's application to join NATO if it doesn't stop supporting the militia group.

"There is too close a connection between these organizations and the PKK ... for it to be good for the relationship between us and Turkey," Swedish Foreign Minister Tobias Billstrom told public service broadcaster Swedish Radio.

"The primary objective is Sweden's membership in NATO," he said.

The move comes just days before Prime Minister Ulf Kristersson is due to travel to Ankara to try to convince Turkey's President Tayyip Erdogan to let Sweden join the military alliance.

Sweden and Finland applied to join NATO earlier this year as a direct consequence of Russia's invasion of Ukraine.

The application has been approved by 28 of NATO's 30 countries. The Nordic countries said this week they were optimistic Hungary would also drop its objections.

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