NATO chief Jens Stoltenberg has hit back against “any suggestion” countries within the alliance would not defend one another after former U.S. President Donald Trump said he would not abide by the collective defense clause at the heart of the alliance if reelected.

In what would be a stunning abandonment of a decades-long core U.S. commitment, Trump, who is running for re-election in November, said during a campaign event Saturday he would encourage Russia to do “whatever the hell they want” to any NATO member country that doesn’t meet spending guidelines and would not offer such a country U.S. protection.

In a statement, Stoltenberg said such comments put European and American soldiers at an increased risk.

“Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk,” Stoltenberg said.