Defense Secretary Leon Panetta said there would be no "unilateral" reductions in the US nuclear arsenal, after President Barack Obama said his country had more of the weapons than it needed.

"The president is always interested in trying to see what we can do to reduce our nuclear arsenal," Panetta told reporters Monday on a flight to Ottawa, where he was to meet with his Canadian and Mexican counterparts.

"We've gone through a nuclear review and presented options to him, but let me be very clear that those options are in no way unilateral. They're all based on potential bilateral negotiations with the Russians."

Speaking at a nuclear summit in South Korea earlier on Monday, Obama had said that -- with more than 1,500 deployed nuclear weapons and 5,000 warheads -- the United States had "more nuclear weapons than we need".

"I firmly believe that we can ensure the security of the United States and our allies, maintain a strong deterrent against any threat, and still pursue further reductions in our nuclear arsenal," Obama said.

Obama said the United States would seek talks with Russia on reducing not only strategic nuclear warheads, but also tactical weapons meant for use on the battlefield and warheads in reserve.

He said he would discuss the issue with Russia's incoming President Vladimir Putin when they meet in May.

A new Strategic Arms Reductions Treaty (START), which entered into force in 2011, limits each side to 1,550 deployed warheads and 700 deployed long-range missiles -- including those fired from submarines -- and heavy bombers.

The treaty, which replaced a 1991 accord that expired in 2009, is seen as one of Obama's most significant foreign policy achievements.