CIA director Mike Pompeo is warning that North Korea is on the cusp of being able to hit the US with a nuclear missile.
He stressed Washington still preferred diplomacy and sanctions but said military force remained an option.
North Korea claims it already has the capability to strike the US.
This picture from North Korea's KCNA - taken on 29 August 2017 - reportedly shows the Hwasong-12 missile lifting off from an undisclosed location near Pyongyang. Photo: AFP / KCNA via KNS
Mr Pompeo also said that a US-Canadian hostage couple freed last week had in fact been held in Pakistan, rather than Afghanistan as initially assumed.
He said Canadian Joshua Boyle and his US wife Caitlan Coleman had been "held for five years inside of Pakistan".
This contradicted accounts from Pakistani officials, who said the family had been held in Afghanistan, and moved across the border into the Kurram tribal district of Pakistan on 11 October.
North Korea is "close enough now in their capabilities that from a US policy perspective we ought to behave as if we are on the cusp of them achieving that objective," Mr Pompeo said at the Foundation for Defense of Democracies, a conservative Washington think tank, on Thursday.
"They are so far along in that, it's now a matter of thinking about how do you stop the final step."
He warned Pyongyang's missile expertise was now advancing so quickly that it was hard for US intelligence to be sure when it would succeed.
"When you're now talking about months our capacity to understand that at a detailed level is in some sense irrelevant," he said.
Last weekend, US Secretary of State Rex Tillerson had also insisted President Donald Trump wanted to resolve the confrontation with North Korea through diplomacy.
His statement had come after Mr Trump had publicly told him not to waste time seeking talks with Kim Jong-un.
In his speech, the CIA director also commented on the return of hostages Joshua Boyle and Caitlan Coleman.
The couple were kidnapped while backpacking in Afghanistan in 2012 and had three children while in captivity.
Mr Pompeo said they had been "held for five years inside of Pakistan" - contradicting the official story given by Pakistani authorities that the couple had been held by the Taliban in neighbouring Afghanistan, and were recently moved across the border into Pakistan.
His comments reinforce US media reports of anonymous US officials claiming the hostages had in fact been held by a Pakistani militant group supported by the country's secret service the entire time.
The Haqqani militant group is regarded as a close Taliban ally with support from Pakistan's military-run intelligence service. Pakistan denies such claims.
"I think history would indicate that expectations for the Pakistanis willingness to help us in the fight against radical [Islamist] terrorism should be set at a very low level," Mr Pompeo said. "Our intelligence would indicate the same."
"I think we should have a very real conversation with them about what it is they're doing and what it is they should do and the American expectations for how they should behave," he added.
US Secretary of State Rex Tillerson is due to travel to Pakistan next week.
In his wide-ranging speech, Mr Pompeo also said that the intelligence community had concluded "that the Russian meddling that took place did not affect the outcome of the [US] election".
The CIA later issued a statement that appeared to contradict Mr Pompeo's remark.
"The intelligence assessment with regard to Russian election meddling has not changed, and the Director did not intend to suggest that it had," Dean Boyd, director of the CIA's office of public affairs, said.
The public report by the Director of National Intelligence released in January stated that the intelligence community did "not make an assessment of the impact that Russian activities had on the outcome of the 2016 election."
US intelligence agencies believe Russia tried to sway the election in favour of Trump and now there are several investigations looking into whether anyone from his campaign helped.
However, they have not assessed whether alleged Russian interference affected the election outcome.
Russia denies the allegations and President Trump has denied any collusion between his campaign and Russian officials.
The allegations are currently investigated by several US parliamentary committees as well as a special counsel, former FBI director Robert Mueller.
- BBC
