WASHINGTON (AP) – The Director of National Intelligence says attempts to deter foreign hackers don’t work, despite President-elect Donald Trump’s plans for a buildup of defensive cyber capabilities.

James Clapper told the Senate Armed Services Committee during a hearing Thursday that the U.S. cannot put stock in what he called cyber deterrence. He says cyber weapons are different than nuclear weapons because they are ephemeral and difficult for enemies to see and evaluate. Clapper says that makes it hard to create what he calls “the substance and psychology of deterrence.”

During the presidential campaign, Trump said he will develop offensive cyber capabilities needed to deter attacks and, if necessary, to respond appropriately.