The U.S. Treasury is imposing sanctions against North Korea's Foreign Trade Bank, the country's main foreign exchange institution, for its role in supporting Pyongyang's nuclear weapons program, President Barack Obama's national security adviser said on Monday.

In a speech to the Asia Society in New York, the White House aide, Tom Donilon, also said China should not conduct "business as usual" with North Korea while Pyongyang threatens its neighbors.

"The United States will not accept North Korea as a nuclear state," Donilon said in prepared remarks. "Nor will we stand by while it seeks to develop a nuclear-armed missile that can target the United States."

He said Washington was willing to negotiate with North Korea but Pyongyang must first take "meaningful steps" to meet its international obligations.

The State Department said on Monday it was designating for sanctions three individuals tied to North Korea's nuclear activities: Pak To-Chun, who heads the agency that manages North Korea's weapons production and arms exports; Chu Kyu-Chang, who directs that agency, the Munitions Industry Department; and O Kuk-Ryol, who is vice chairman of the North Korean National Defense Commission.

The Treasury Department on Monday also imposed sanctions on Paek Se-Bong, chairman of North Korea's Second Economic Committee, which oversees the production of North Korea's ballistic missiles.

North Korea uses the Foreign Trade Bank "to facilitate transactions on behalf of actors linked to its proliferation network, which is under increasing pressure from recent international sanctions," the Treasury said in a statement.