WASHINGTON (CNN) -- U.S. Secretary of State Hillary Clinton had strong words Wednesday for North Korea's nuclear activities and saber-rattling, saying the secretive communist nation "has ignored the international community" and "continues to act in a provocative and belligerent manner toward its neighbors."

An image from North Korean television on April 9 shows leader Kim Jong Il in Pyongyang.

"There are consequences to such actions," Clinton said.

Also Wednesday, a nonprofit institution that focuses on stopping the spread of nuclear weapons countered news reports that North Korea "may have begun reprocessing spent nuclear fuel at its plutonium separation plant at Yongbyon."

The Washington-based Institute for Science and International Security said the reports apparently refer to "recent classified U.S. imagery which reportedly show steam present at the reprocessing facility."

But the group issued a statement Wednesday saying that commercial satellite images taken Tuesday don't show steam from pipes running from a nearby coal-fired plant to the reprocessing facility.

"The May 26 imagery also does not show any smoke from the chimney at the coal-fired plant, nor any plume from the stacks at the reprocessing plant," the statement said. "North Korea announced in April that it intended to reprocess spent fuel at the facility. It is difficult to know when that reprocessing will start or finish."

The group added that "there also does not appear to be any construction activity at the site of the destroyed cooling tower for the [5 megawatt] reactor at Yongbyon." The cooling tower was disabled last year in an implosion. Watch Clinton warn North Korea »

Still, a senior administration official confirmed Wednesday that there has been increased activity at the facility, including the restarting of generators to create steam for reprocessing. It would take months to fully restart the whole complex, but the preparations are still considered serious. Watch the story of the DMZ, which splits North and South »

Clinton said North Korea has chosen to violate "specific language of the U.N. Security Council resolution 1718" and "abrogated obligations it entered into through the six-party talks." The U.N. resolution -- which had been adopted in 2006 after North Korea's first nuclear test -- condemned the test and imposed sanctions on the country.

Clinton answered a question about the issue from CNN during an appearance with the Egyptian foreign minister.

"They have chosen the path they are on and I am very pleased that we have a unified international community, including China and Russia, in setting forth a very specific condemnation of North Korea and working with us for a firm resolution going forward," Clinton said, also underscoring the commitments the United States "has and intends to honor" toward South Korea and Japan.

North Korea conducted a nuclear test on Monday and fired five short-range missiles Monday and Tuesday. The country threatened military action Wednesday after South Korea joined a U.S.-led effort to limit the trafficking of weapons of mass destruction.

In Washington, a State Department spokesman didn't elaborate Wednesday on the reports that North Korea had resumed operating its Yongbyon nuclear facility.

"We're aware of the reports," said spokesman Ian Kelly. "I'm just not going to comment on any intelligence matters."

Pressed for a reaction from reporters, Kelly said North Korea "should refrain from any provocative actions -- and clearly restarting the nuclear reactor would be a provocative action -- and uphold the commitments that they've made."

A senior U.S. defense official told CNN that air samples taken from international airspace after the nuclear test are en route to the United States for testing. The State Department will announce the results when they come in, probably by the end of the week, the official said.

Clinton said there are discussions in the United Nations and the international community to determine how to proceed in dealing with the issue.

"We hope there will be an opportunity to come back into a framework of discussion within the six-party process," she said. The six-party process -- including the nations of North Korea, South Korea, the United States, Russia, China and Japan -- is the diplomatic effort to address North Korea's nuclear program.

Since the April launch of a North Korean rocket, Pyongyang has considered almost any opposition a "declaration of war," including U.N. Security Council sanctions and participation in the U.S.-led Proliferation Security Initiative.

Within two weeks of the April 5 launch, the Security Council adopted a declaration condemning North Korea for the step. The North Korean Foreign Ministry said the condemnation infringed on the nation's sovereignty and amounted to a declaration of confrontation and war.

"[North Korea's] revolutionary armed forces will opt for increasing the nation's defense capability, including nuclear deterrent, in every way, without being bound to the agreement adopted at the six-party talks," the Foreign Ministry continued.

After Monday's nuclear test by North Korea, the Security Council condemned the move as a "clear violation" of international law. Even Pyongyang's closest ally, China, criticized the exercise, saying North Korea "disregarded the opposition of the international community."

Along with conducting the nuclear test, the North has fired five short-range missiles this week -- two Monday and three Tuesday -- according to Won Tae-jae, a spokesman for South Korea's Ministry of National Defense, the South's Yonhap news agency said.

North Korea's actions have heightened tensions worldwide, though U.S. officials said other nations will not be intimidated by the "provocative and destabilizing" moves, particularly Monday's nuclear test.

After passing a nonbinding statement of criticism Monday, the Security Council is now working on passing "a strong resolution with teeth," said U.S. Ambassador to the U.N. Susan Rice.

"Those teeth could take various different forms -- there are economic levers, there are other levers that we might pursue," she said.

North Korea first tested a nuclear weapon in October 2006. Pyongyang threatened last month to carry out a new test after the Security Council reacted to its test-firing of a long-range rocket by extending economic sanctions against the nation, which desperately needs food and energy assistance.

North Korea agreed in 2008 to scrap its nuclear weapons program -- which it said had produced enough plutonium for about seven atomic bombs -- in exchange for economic aid. But the deal foundered over verification and disclosure issues, and the North expelled international inspectors and announced plans to restart its main nuclear reactor at the Yongbyon complex.

CNN's Barbara Starr contributed to this report.

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