WASHINGTON - From Beijing to Washington, world leaders weighed their difficult options Monday for responding to claims by North Korea that it had conducted an underground test of a nuclear weapon, with the U.S. proposing severe sanctions against the isolated communist nation.

As the government of President Kim Jong Il faced almost unanimous condemnation for the test - including from North Korea's main ally and economic backer, China - the United Nations Security Council demanded that it refrain from further tests, return to stalled negotiations and end its weapons program.

The Bush administration circulated a draft U.N. resolution late Monday that would ban North Korean ships and aircraft from other countries, prohibit all trade in military and luxury goods with Kim's government, prevent its "abuses" of the international financial system for the development of banned weapons and impose travel restrictions on North Korean officials.

President Bush strongly condemned the reported underground explosion, saying he had consulted with the leaders of South Korea, Japan, China and Russia.

"All of us agreed that the proclaimed actions taken by North Korea are unacceptable and deserve an immediate response by the United Nations Security Council," Bush said.

The president also issued a warning to Kim's government, which is known to have sold missile technology to Pakistan and Iran, among other nations.

"The transfer of nuclear weapons to states or non-state entities would be considered a grave threat to the United States," Bush said. "And we would hold North Korea fully accountable to the consequences of such action."

North Korea's audacious test came after the government announced last week that it would carry out such an action, and after a weekend of speculation by Japanese officials and others that test preparations were well under way in the country's mountainous north.

Top administration aides, including Secretary of State Condoleezza Rice and National Security Adviser Stephen Hadley, informed Bush of the test and monitored it late into the night, according to White House spokesman Tony Snow. The test took place about 10:36 a.m. Monday, according to South Korean officials.

Despite North Korea's claims, the U.S. and other nations were still trying to confirm that the blast in North Korea's North Hamgyeong province was indeed a nuclear device. And some doubts lingered over how large or successful the reported test was, and what it means for how close North Korea is to having a real weapon.

A Russian official said that seismic monitoring equipment recorded an underground blast that was the equivalent of 5,000 to 15,000 tons of TNT - about the force of the 1945 bomb the U.S. dropped on Hiroshima, Japan. But South Korea said its equipment measured a far smaller explosion, equal to about 500 tons of TNT.

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