The Obama administration has called for a peaceful and stable leadership transition in North Korea, but made few demands on a nuclear-armed nation known for its unpredictability, poverty and hostility to the United States.
Prospects for new nuclear disarmament talks involving North Korea and the United States appeared to dim with the unexpectedly sudden death of North Korean leader Kim Jong Il and uncertainty surrounding the planned succession to his politically untested son.
Top Obama administration national security officials are focusing intelligence and other assets on the opaque internal politics of the reclusive communist nation that George Bush once placed on an "axis of evil" enemies list.
Barack Obama conferred by phone with Japanese Prime Minister Yoshihiko Noda to underscore the US commitment to Japan and other close allies, the White House said, and also conveyed the importance he placed on stability in the region.
Mr Obama also spoke to South Korean president Lee Myung-bak, and the administration also contacted officials in China and Russia following the news of Kim's death, the White House said.
"We are deeply concerned with the well-being of the North Korean people and our thoughts and prayers are with them during these difficult times," US secretary of state Hillary Clinton said.
"It is our hope that the new leadership of the DPRK will choose to guide their nation on to the path of peace by honouring North Korea's commitments, improving relations with its neighbours, and respecting the rights of its people."
Mrs Clinton did not say how Kim's death would affect the US approach to his country, nor did she make any demands on the new leadership, passing up the opportunity to reiterate long-standing US calls for North Korea to follow through on previous nuclear disarmament pledges. The omission of what has been a standard element of any US officials' comments on North Korea appeared to underscore Washington's concern about the situation.
The State Department later said it still was the US view that North Korea make good on those commitments. But the department said Kim's passing and assumption of power of his son, Kim Jong Un, would delay anticipated developments on resuming nuclear disarmament talks with the North and supplying the nation with food aid.
Meanwhile President Hu Jintao visited North Korea's embassy in Beijing to offer his condolences as China moved swiftly to assure its communist ally of its strong support amid an uncertain leadership transition. The official Xinhua News Agency reported the visit but offered no other details.