The United Nations Security Council is considering several US proposals for responding to North Korea's apparent nuclear test.
The Council already condemned the test in general terms, but UN legal experts meet later Monday to consider specific actions.
Britain has said it will push for sanctions against North Korea under Chapter Seven of the UN Charter -- a section that governs sanctions or military force in response to threats to peace or acts of aggression.
The United States has proposed inspecting all cargo going into or out of North Korea to check for weapons of mass destruction or related materials.
North Korea's UN ambassador Pak Gil Yon said the Security Council should congratulate the North instead of passing what he called "notorious, useless" resolutions.
International monitors confirmed that a seismic tremor detected Monday morning in northeastern North Korea suggested a nuclear blast.
US intelligence officials estimated the blast's size at less than one thousand tons of TNT, and said they have not yet determined if it was nuclear.
Russian officials estimated the blast may have measured as much as 15-thousand tons of TNT -- about the same size as the atomic bomb the United States dropped on Hiroshima, Japan, in 1945.
South Korean intelligence officials said there are indications the North may be preparing another test.
The White House said that early Monday, North Korea notified China of an upcoming nuclear test. Chinese officials then passed the message on to the United States.
Seismic monitors later picked up signals of a blast and North Korea announced on state television that the nuclear test was carried out successfully and safely, with no radiation leakage.
South Korea has raised its military alert level, and its president convened an emergency security meeting. The Korean peninsula has been particularly tense since Saturday, when South Korean forces fired at North Korean troops who crossed the Demilitarized Zone.