WASHINGTON -- US President Donald Trump's policy on North Korea is "failing" and the communist nation poses an imminent threat to American troops and allies, one of his former national security advisers said Monday.

John Bolton, who worked for Trump until September, made the remark in a tweet as tensions have risen over North Korea's threat to send an unwelcome "Christmas gift" to the United States in protest of their stalled denuclearization talks.

"We say that it's unacceptable for North Korea to obtain nuclear weapons, but that's more rhetorical than a real policy at this point," wrote Bolton, who left the Donald Trump administration in September after numerous policy clashes with the US president, including over North Korea.

"The risk to US forces & our allies is imminent & more effective policy is required before NK has the technology to threaten the American homeland," he said.

Bolton posted a link to an interview he gave to Axios, in which he talked about the Trump administration's "failing policy" in North Korea and the possible need to admit it was wrong.

"The idea that we are somehow exerting maximum pressure on North Korea is just unfortunately not true," Bolton said in the interview.

In an interview with NPR last week, Bolton reiterated his view that he does not think North Korea will ever give up its nuclear weapons voluntarily. (Yonhap)