A top White House drug policy official is threatening retaliation from the U.S. if Canada relaxes its laws against marijuana possession.

David Murray, right-hand man to U.S. "drug czar" John Walters, says he doesn't want to tread on another country's sovereignty, but warned there would be consequences if Canada proceeds with a plan to decriminalize the possession of marijuana.

"We would have to respond. We would be forced to respond," said Murray.

Murray didn't spell out what the American response would be, but he invoked images of tie-ups at border crossings and intense bureaucracy.

He said if marijuana becomes more widely used in Canada, it could penetrate more widely into the U.S.

Murray tried to express the feeling in the U.S. that looser drug laws go hand-in-hand with an increase in crime and drug addiction among youth, and used some apocalyptic language to do it.

"You can't wall this off saying, 'We're only talking about a little cannabis.' Our experience is they come together like the Four Horsemen," he said.

Murray said Canada's reputation in the global community would be forever altered if it decided to decriminalize pot.

"It's not just Canada's relationship with the United States that would change; it's Canada's relationship with the world," he said.

In fact, many countries, notably in Europe, have already decriminalized marijuana, but none of them share a border with the U.S., where the policy is zero tolerance for smoking pot.