European Commission President Jean-Claude Juncker said on Thursday that Europe must not cave in to US demands to raise military spending, arguing that development and humanitarian aid could also count as security.

US president Donald Trump has raised questions about his commitment to the NATO defence alliance if European countries do not raise defence spending to 2 per cent of economic output.

The United States puts up 70 per cent of alliance funds.

US Defence Secretary Jim Mattis warned North Atlantic Treaty Organisation allies on Wednesday that they must honour military spending pledges to make sure the United States does not moderate its support.