North Korean leader Kim Jong Il, 69, has reportedly died after nearly two decades in power.
The White House said it was closely monitoring reports that the reclusive leader was dead.
President Obama has been notified of the reports, and the U.S. is in touch with South Korea and Japan, the White House said in a statement Sunday night.
Kim's death introduces new uncertainties into the stability of Asia, where Obama has sought to advance U.S. economic and security interests. Asian stock indexes fell with the news that he had died.
Japan and South Korea's governments both held meetings of their national security councils, according to reports. South Korea's military was placed in a state of high alert.
Kim had been in control of North Korea since his father died in 1994, and is believed to have positioned his own son, Kim Jong Un, to take over.
North Korea has been a thorn in the side of U.S. administrations for decades as it has sought to develop a nuclear arsenal. Former President George W. Bush famously included it in his "axis of evil" speech along with Iran and Iraq.
Kim sought nuclear power while often struggling to feed his own people, who have little contact with the world outside the northern part of the Korean peninsula.
Former U.S. Ambassador to the United Nations Bill Richardson on Monday said there should be "extreme concern" after the death of Kim Jong Il, and characterized the Korean peninsula as a "tinderbox."
Richardson said there had recently been a "thawing" in talks with the regime and questioned whether that would continue under new leadership. Richardson said the next "48 hours" would reveal if the country's generals intended to "coalesce" around Kim Jong Un.
Republican Sen. Mark KirkMark KirkThe way forward on the Iran nuclear deal under President Trump ObamaCare repeal bill would defund Planned Parenthood Leaked ObamaCare bill would defund Planned Parenthood MORE (Ill.) said the U.S. and its allies were "entering into a shaky time."
"It's a nuclear armed country that also sells its missiles around the world," Kirk said Monday on "Fox & Friends."
Kirk expressed concern that presumptive heir Kim Jong Un, whom he said was "only 28 or 29 years old," might "try to make a move on South Korea ... to look tough."
Kirk, however, praised Obama, saying that the president had done "the right thing" by calling South Korea's leaders and reaffirming American support.
The death of the man known in North Korea as the "dear leader" is also sure to prompt discussion in the 2012 presidential race about how to handle that country's future.
Mitt Romney said Kim's death "represents an opportunity for America to work with our friends to turn North Korea off the treacherous course it is on and ensure security in the region."
"America must show leadership at this time," he added.
Jon Huntsman, a former ambassador to China and contender for the GOP presidential nomination, said Kim Jong Il's death "closes a tragic chapter for the people of North Korea and offers them the best opportunity to get on a path towards a more free and open society and political reform" in a statement released Monday.
Huntsman called on the U.S. to "pay extremely close attention to the disposition of North Korea's nuclear bombs, nuclear materials, and other elements of their WMD program."
This story was posted on Dec. 18 at 11:40 p.m. and has been updated.