ISTANBUL: Turkish President Recep Tayyip Erdogan has reaffirmed that his country would not green-light Sweden's accession to NATO unless the Nordic nation adopted a firm stance against "terror groups".

"How can a state that doesn't distance itself from terror organizations contribute to NATO?" Xinhua news agency quoted Erdogan as saying at the graduation ceremony of the National Defense University in Istanbul on Friday.

He said his government expected Sweden to abide by its promise of taking into account Turkey's security concerns.

The President further said that he would bring that to allies' attention at the NATO summit held in Vilnius, Lithuania next week.

Erdogan noted that Turkey backed NATO's open-door policy and was ready to welcome any state committed to the alliance's fundamental principles.

Sweden and Finland applied to join NATO last year but faced objections from Turkey on the grounds that the two countries harbor members of the outlawed Kurdistan Workers' Party and the Gulen movement.

Turkey eventually lifted its objection to Finland earlier this year after the Nordic country took "concrete steps" against such groups, and the country went on to become NATO's 31st member in April.

But Ankara maintains its veto of Sweden.

The military alliance will hold a summit in Vilnius, Lithuania, on July 11-12, at which NATO wants to finalise Sweden's admission process.