North Korea should look to Myanmar as proof of America's willingness to forgive and build a new era of relations, a top US advisor said Monday, even as new sanctions were imposed on Pyongyang.

US President Barack Obama was willing "to offer his hand to those who would unclench their fists," National Security Advisor Tom Donilon said.

Despite three nuclear tests and several missile tests, the United States was open to "authentic negotiations" with North Korea to help "develop its economy and feed its people -- but it must change its current course."

"We ask only that Pyongyang prove its seriousness by taking meaningful steps to show it will abide by its commitments, honor its words, and respect international law," Donilon said.

His comments to The Asia Society in New York came as North Korea seemed to make good on its threat to tear up the 60-year-old armistice agreement, which ended the Korean war in 1953, and also apparently cut off a hotline with Seoul.

Donilon urged new leader Kim Jong-un to take Washington at its word, saying he only needed to look at what has happened in the past year or so in Myanmar since the military leader there launched a slew of democratic reforms.

"Anyone who doubts the president's commitment needs look no further than Burma," Donilon said, using the former name for the Southeast Asian nation.

Obama's visit to Yangon in November "is proof of our readiness to start transforming a relationship marked by hostility into one of greater cooperation," he insisted.

"Burma has already received billions in debt forgiveness, large-scale development assistance, and an influx of new investment," the top official said, adding that now Myanmar had "broken out of isolation and opened the door to a far better future for its people."

But he also insisted that Washington would defend itself and its allies from any threat from Pyongyang, after the isolated nation threatened a pre-emptive nuclear strike to retaliate against new UN sanctions agreed following last month's nuclear test.

The United States also moved to punish North Korean leaders for last month's nuclear test by slapping sanctions the Foreign Trade Bank of North Korea, "for its role in supporting North Korea's WMD program."

The sanctions on Pyongyang's main foreign exchange bank as well as against three top members of the North Korean government is aimed at ensuring that the country's elite begins to feel them bite.

Donilon, who also announced that South Korea's new President Park Geun-Hye would hold White House talks with Obama in May, warned North Korea that the United States stood firm with Japan and South Korea.

"The days when North Korea could exploit any seams between our three governments are over," he said.

China also has a key role to play, he stressed, welcoming Beijing's support for the tightened UN sanctions against Pyongyang agreed last week.

"We believe that no country, including China, should conduct "business as usual" with a North Korea that threatens its neighbors," Donilon said.

"China's interest in stability on the Korean Peninsula argues for a clear path to ending North Korea's nuclear program."