The leaders of Japan and China met Sunday for the first time in five years.
New Japanese Prime Minister Shinzo Abe is on a fence-mending mission, trying to improve relations with China and South Korea.
Followinghis meeting in Beijing with ChinesePresident Hu Jintao, Abetold reporters that the prospect of North Korea undertaking its first nuclear test would be"a great threat to East Asia and the international community."
The two leaders also called for North Korea to return to the six-party negotiations, which it left a year agoafter a disagreement with the United States.
Hours later, North Korea's official news agency announced that the communist nation had conducted the test early Monday.
North Korea said last weekit would test an atomic weapon because of"the U.S. extreme threat of a nuclear war," but did not say when.
Mission to improve relations
Abe, who became prime minister last month, istrying toimprove relations with China and South Korea, which drew away from Japan because of former PM Junichiro Koizumi's visits to the Yakusuni Shrine, where Japan's war dead are honoured.
Among the dead are a handful of men who committed war crimes against China and South Korea in the Second World War. That angers both countries, and China broke off high level visits in 2001 because of the visits.
Japan wants sanctions
Abewants to improve economic relations with China, andis also pushing both of Japan's neighbours to toughen their stance against North Korea.Japan and the United States have imposed sanctions —Japan after North Korea test-fired missiles during the summer —but China and South Korea have rejected that course of action.
The UN Security Council warnedNorth Korea tostop its test plans last week, andJapan's Foreign Ministry said it is ready to demand strongmeasures at the UNif the test goesahead.
Abe is scheduled to meetSouth Korean President Roh Moo-hyun on Monday.