The defender – who made his first-team debut against Huddersfield Town last week – was set to be called up by Hungary for the prestigious Under-20 World Cup in Egypt later this month.
However, with the tournament spanning three weeks, and Newcastle threadbare in the centre of defence, caretaker boss Chris Hughton has asked Kadar to remain on Tyneside as cover to Steven Taylor and Fabricio Coloccini.
Kadar: "Newcastle said I can't go.
"I will stay here and do my best because my club is Newcastle, and if Newcastle say, 'stay here', then I stay here.
"It's the World Cup for the Under-20s, which is for three weeks.
"It's a long time to be away from here, and I want to play in the Newcastle first team. If I go home, I'll do my best there, but if I stay here, then I'll do my best for Newcastle.
"Wherever I am, I want to play."