Gates told US lawmakers at a hearing Tuesday that the US would have a "high probability of being able to defend ourselves" if the North fired a long-range missile.
He added that US missile interceptors "are fully adequate to protect us from the North Korean threat for a number of years."
Gates also noted that if the North Korean "threat" should change, the United States could easily expand its missile interceptor force.
'We have got makings of deal'
Meanwhile, the five permanent members of the UN Security Council are close to agreement on a new resolution imposing further sanctions on North Korea over its nuclear weapons program, a diplomat close to the talks said on Tuesday.
The United States and Japan have been pushing for strong sanctions to punish North Korea for its nuclear test in May, but China and Russia have been cautious about more sanctions.
"We have got the makings of a deal, but one delegation still needs to hear back from its capital," the diplomat said on the sidelines of talks at the United Nations on a draft resolution.
Two diplomats from among the permanent five Security Council members said they believed the United States and China had agreed on the text of a resolution within the last 24 hours, but Russia had raised a concern.
AP and Reuters contributed to the report