An alien encounter may not be a pleasant one and it is probably unwise for human beings to be advertising our location, an American Nobel Prize winner says.

"I think it is probably not the smartest thing to tell the aliens where we are, as any encounter with aliens may not be a happy one," Brian P. Schmidt told the 28th General Assembly of the International Astronomical Union in Beijing, China's Xinhua news agency reported.

Schmidt shared the 2011 Nobel Prize in physics for providing confirming evidence the expansion of the universe is accelerating.

"Aliens may not be something that we need to worry about," he said. "It will be so far away and it takes so long to travel from point A to point B in the universe that it won't be a problem.

"But it will happen when it happens."

It will be difficult to reach another planet in a universe expanding at an ever-accelerating rate, he said, meaning we are less likely to meet higher intelligent extraterrestrial life in the future.

"The future of the universe seems to be dark. Things are getting faster and faster. In terms of looking for aliens, it's gonna be quite a challenge.

"It may never happen. Things like us are probably very rare in the universe," he said.