Keane, a retired four-star general and current chairman of the Institute for the Study of War, declined Trump's offer to serve as secretary of defense, citing personal issues connected with the recent death of his wife from Parkinson's disease.
"A preemptive strike against launch facilities, underground nuclear sites, artillery and rocket response forces and regime leadership targets may be the only option left on the table," Keane cautioned in an interview with the Times of London.
"We are rapidly and dangerously moving towards a military option."
The remarks echo those Trump himself made, ahead of his meeting with Chinese President Xi Jinping on Thursday. He said that the US is ready for an alternative, in case China doesn't put enough pressure on Kim Jong-un's regime to abate its nuclear ambitions.
Secretary of State Rex Tillerson has dubbed Pyongyang "an imminent threat," based on its recent actions and significant advances both with nuclear warheads and with ballistic missiles.
While some experts have proposed peaceful means of dealing with the problem, such as having unconditional talks with the North Koreans, or suspending or limiting the annual Foal Eagle military drills between South Korea and the US, others appear to be losing faith in diplomacy.
"Sanctions have not worked against North Korea and I doubt more sanctions will and I don't believe sanctions against China will work any better," Keane said.
"Our last three presidents spanning over 20 years have failed to stop the North Korean nuclear program," he said, adding that he hopes Trump's team will succeed.
