North Korea has restarted its weapons-grade nuclear power plant and fired its sixth short-range missile, news reports said Wednesday, in a growing standoff with world powers following its latest nuclear test.

The missile launches over the past two days came as leaders around the world condemned North Korea for Monday's underground nuclear test and the U.N. Security Council debated possible new sanctions against the communist nation. Retaliatory options were limited, however, and no one was talking publicly about military action.

South Korea's mass-circulation Chosun Ilbo newspaper reported that U.S. spy satellites have detected steam coming from a reprocessing facility at North Korea's main Yongbyon nuclear plant, indicating the North has restarted the reprocessing of spent nuclear fuel rods to harvest weapons-grade plutonium.

Its report quoted unnamed officials. South Korea's Defense Ministry and the National Intelligence Service — the country's main spy agency — said they cannot confirm the report.

The North had said it would begin reprocessing in protest over international criticism of its April 5 rocket launch.

North Korea also test-fired three short-range missiles Tuesday, including one late at night, from the east coast city of Hamhung, according to South Korea's Yonhap news agency. South Korea's spy chief confirmed two other missiles were launched Monday, but reports put the number at three Monday for a total of six.

More could be planned. North Korea has warned ships to stay away from waters off its west coast through Wednesday, suggesting more test flights.

Details of Monday's nuclear test may take days to confirm, but Russian defense officials said the blast was roughly as strong as the bombs dropped on the Japanese cities of Hiroshima and Nagasaki in World War II and was stronger than North Korea's first test in 2006.

In New York, U.N. diplomats said key nations were discussing a Security Council resolution that could include new sanctions against North Korea.

Ambassadors from the five permanent veto-wielding council members — the United States, Russia, China, Britain and France — as well as Japan and South Korea were expected to meet soon, the diplomats said, speaking on condition of anonymity because the meeting is private.

The Security Council met in emergency session Monday and condemned the nuclear test. Council members said they would follow up with a new legally binding resolution.

France's deputy U.N. ambassador Jean-Pierre Lacroix said his government wants a resolution to "include new sanctions ... because this behavior must have a cost and a price to pay."

It was too early to say what those sanctions might be and whether China and Russia, both close allies of North Korea, will go along.

In an unusual step, China strongly reproached its close ally.

Chinese Foreign Ministry spokesman Ma Zhaoxu reiterated that Beijing "resolutely opposed" the nuclear test. It urged Pyongyang to return to negotiations under which it had agreed to dismantle its atomic program.

North Korea is "trying to test whether they can intimidate the international community" with its nuclear and missile activity, said Susan Rice, U.S. ambassador to the United Nations.

"But we are united, North Korea is isolated, and pressure on North Korea will increase," Rice said.

Diplomats acknowledged, however, that there were limits to the international response and that past sanctions have had only spotty results.

North Korea blamed the escalating tensions in the region on Washington, saying the U.S. was building up its forces, and defended its nuclear test as a matter of self-preservation.

An editorial in the North's Rodong Sinmun newspaper called the United States "warmongers" and said Washington's recent announcement about sending fighter planes to Japan "lay bare the sinister and dangerous scenario of the U.S. to put the Asia-Pacific region under its military control."

At the Conference on Disarmament in Geneva, An Myong Han, a diplomat from the North Korean mission, said his country "could not but take additional self-defense measures, including nuclear tests and the test launch of long-range missiles, in order to safeguard our national interest."