"The landscape of nuclear weapons have changed in the last 25 years," Carter said on Tuesday in his remarks on "Sustaining Nuclear Deterrence" at Minot Air Force Base in North Dakota.
"Pakistan nuclear weapons are entangled in a history of tension, and while they are not a threat to the United States directly, we work with Pakistan to ensure stability."
Carter said while the US has not done much to boost its nuclear arsenal, other countries have added to it both in terms of the number of weapons and delivery options.
He also praised India for showing responsible behaviour with its nuclear technology.
Expressing concern over nuclear technology in North Korea, he said, "It is essential that the US maintains its nuclear deterrence."
"America's nuclear deterrence is the bedrock of our security and the highest priority mission of the Department of Defence," he said.
Observing the Russia has long been a nuclear power, Carter said the country's recent saber-rattling and building of new nuclear weapons systems raises serious questions about its leader's commitment to strategic stability and their regard for long-established abhorrence of using nuclear weapons.
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