The United States and its allies must "stand up" to North Korea, President Obama said Monday, accusing the rogue regime of blatantly defying the international community with its nuclear and missile test. 

Obama addressed the issue in the Rose Garden on his way to Memorial Day services at Arlington National Cemetery. He called North Korea's tests "reckless" and a "grave threat" to international security. 

"The United States and the international community must take action in response," Obama said, noting that North Korea had abandoned a previous pledge to halt its nuclear program and has with its actions violated United Nations resolutions. "We will work with friends and allies to stand up to this behavior." 

The communist country said it had carried out a powerful underground nuclear test, much larger than one conducted in 2006. The regime also test-fired three short-range, ground-to-air missiles later Monday from the same northeastern site where it launched a rocket last month, the Yonhap news agency reported, citing unnamed sources. 

The twin moves were roundly condemned throughout Washington Monday. 

"The United States will never waver from our determination to protect our people and the peace and security of the world," Obama said. 

He accused North Korea of increasing tensions and threatening stability in the region, while only further isolating itself. 

"It will not find international acceptance unless it abandons its pursuit of weapons of mass destruction and their means of delivery," Obama said earlier in a written statement. "The danger posed by North Korea's threatening activities warrants action by the international community." 

He said the United States would continue working in multilateral talks and will hold consultations with members of the U.N. Security Council.

Both the missile firing and nuclear test caught U.S. officials by surprise, according to one official, though Obama and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said North Korean leaders' recent statements previewed such a move. 

"They didn't give us any warning whatsoever," a senior U.S. intelligence official who works on North Korean issues told FOX News. 

The tests added an extra layer of urgency to House Speaker Nancy Pelosi's trip to China, where she is meeting with officials to discuss climate change. 

"Such action by North Korea is unacceptable and cause for great alarm," Pelosi said in a written statement from Shanghai. "These reported tests underscore the message our congressional delegation planned to deliver to top Chinese government leaders during our meetings later this week: the Chinese must use their influence to help bring North Korea to the table for the Six-Party talks. Today's announcement makes that need all the more urgent." 

Mullen told FOX News that North Korea's actions speak "to their growing belligerence." 

He said the country is still trying to "destabilize" the region. 

"And in the long run should they develop a nuclear weapons program, and a program that could in fact be put on missiles that could reach the reach the United States, that could be a significant threat to us," he said. 

Former South Korean Foreign Minister Han Sung-Joo told FOX News his scientists estimated the scale of the latest North Korean nuclear test to be about 10 times the size of the 2006 nuclear test. 

They registered the tremor from the blast at 4.5 on the Richter scale. That is 1.0 more than in 2006, or 10 times greater. It is estimated that the 2006 test was about 800 tons, just under one kiloton. That would make today's blast around 10 kilotons. 

By comparison, the bomb dropped on Hiroshima was 22 kilotons. 

The former South Korean foreign minister called the latest test serious, and a potential "game-changer." 

He said North Korea's Kim Jong Il is trying to escalate its nuclear capabilities to attain a better bargaining position, and eventually is seeking bi-lateral talks with the United States. He said North Korea is directly trying to draw attention from the Obama administration. 

FOX News' James Rosen and Greg Palkot and The Associated Press contributed to this report.