"I think this is an important goal for everyone to have in the world, but I think that it's a long road to get there," he told CNN.

"President Obama is the fourth president that I have worked for who has said publicly he would like to see an end to nuclear weapons and (have) a nuclear weapons-free world. I think that's a laudable objective," Gates said.

But the US defense chief said any such move would be as the result of gradual and labored disarmament efforts.

"It's a goal that you have to move toward step by step," Gates said.

Obama made his call for a nuclear-free world last month in Strasbourg, France, on the margins of a North Atlantic Treaty Organization (NATO) summit, when he announced his intention week to "seek the goal of a world without nuclear weapons."

Gates noted Sunday that "we have had a number of countries forego nuclear weapons, countries that ... really voluntarily walked away from them: South Africa, Libya, Taiwan, South Korea, Argentina, Brazil.

"So total pessimism with respect to nonproliferation, I think, is unwarranted."

But he said the spread around the world of nuclear know-how will make it difficult to ever fully eradicate such weapons.

"How do you deal with the reality of that technology being available to almost any country that seeks to pursue it?" Gates told CNN.

"And what conditions do you put in place, what UN verification measures or IAEA (International Atomic Energy Agency) verification measures do you put in place, to prevent others from getting that?"

Still, Gates said, the United States should continue ongoing proliferation efforts, including talks on a Comprehensive Test Ban Treaty, and negotiations with Russia on new post-START talks aiming to reduce the US-Russia nuclear stockpile.

"These are all important steps in that direction. But my guess is, it's a long march," he said.