A White House official made two secret visits to North Korea last year in an effort to improve relations after new ruler Kim Jong-Un assumed power, former US officials say.
The brief visits in April and August were aimed at encouraging the new leadership to moderate its foreign policy after the death of Kim's father, longtime autocrat Kim Jong-Il, in December 2011.
The ruling elite apparently spurned the outreach effort, however.
This month, after a surge of fierce anti-US rhetoric, the government in Pyongyang defied international warnings and conducted its third and most powerful underground nuclear test.
The former US officials, who spoke on condition of anonymity because the back-channel trips have not been formally disclosed, said the first visit was an unsuccessful attempt to persuade North Korea not to launch a long-range rocket.
North Korea carried out the launch on April 12.
The missile flew only a few minutes before it exploded and crashed into the sea.
A subsequent test of another long-range rocket in December was successful.
The April trip was led by Joseph DeTrani, a North Korea expert who then headed the National Counter Proliferation Center in the Office of the Director of National Intelligence, which coordinates US intelligence agencies, the former US officials said.
It was unclear who led the August trip.
They said Sydney Seiler, who is in charge of Korea policy at the National Security Council, went on both trips.
Seiler, a veteran CIA analyst, speaks fluent Korean.
He could not be reached for comment.
The White House, State Department and CIA have refused to confirm or deny the 2012 trips, which occurred during the US presidential election season.
"I'm not going to comment on this," Tommy Vietor, spokesman for the National Security Council, said in an email.
US officials have visited North Korea on and off for years.
They include Secretary of State Madeleine Albright, who led an official state visit in 2000.
The last official US visit was in 2009 when US special envoy Stephen Bosworth sought to restart stalled six-party negotiations on North Korea's nuclear program.
The talks have not resumed.
DeTrani left the government last year and now heads the Intelligence and National Security Alliance, an industry group.
"There are certain things I just won't talk about, and this is one subject I really feel it's not appropriate for me to comment on," DeTrani said in a telephone interview.
Without confirming the 2012 trips, DeTrani said it "makes eminent sense" for the United States to conduct talks with North Korean officials after Kim Jong Il's death.
DeTrani said he and other US experts initially saw signs that Kim Jong-Un might behave less rigidly than his father, including putting moderate figures in key government positions.
Those hopes were quickly dashed, however.
In addition to the rocket launches and nuclear test, the new leader appointed a defence minister, General Kim Kyok-Sik, who reportedly was responsible for the 2010 shelling of a South Korean island that killed four people, and the sinking of a South Korean naval ship that killed 46 sailors.
"I was initially guardedly optimistic that (Kim Jong-Un) was moving in the right direction," DeTrani said.
"With the launches and the test, he's reversed that."
News of the secret trips first leaked out in the South Korean media, which said the flights from Guam to the Sunan airport in Pyongyang, North Korea's capital, passed through South Korean airspace.
"The South Korean air force was tracking the plane. They knew there was a special flight going to Pyongyang, but the purpose of the mission was a secret," said Moon Chung-in, a former South Korean government adviser on North Korea and a professor at Yonsei University in Seoul.
Some North Korea specialists applauded the administration of Barack Obama for breaking a long-standing taboo against one-on-one talks with Pyongyang.
"The trips were a good idea and I think the fact that they did them secretly was a good idea," said John Delury, a North Korea expert at Yonsei University.
He visited Pyongyang in January on a private trip with Google's chairman, Eric Schmidt.
"I don't know why at this point the administration just doesn't set the record straight on this," said a former US official.
"All it shows is that we were trying to walk the last mile with North Korea."
Another analyst said disclosing the trips would subject them to scrutiny from Japan and South Korea, as well as congress.
Chuck Downs, a former Pentagon official and Korea expert, characterised the visits as a worthy gambit.
"The history of this past year of seemingly unsuccessful efforts to seek common ground on which to deal with Kim Jong-Un will, I suspect, be a very important study in foreign relations for years to come," he said in an email.