The UN Security Council has "strongly condemned" North Korea's third nuclear test.
South Korean Foreign Minister Kim Sung-hwan, whose country is president of the council this month, called the test "a grave violation of Security Council resolutions."
Speaking after an emergency meeting on February 12, he said the council would now consider "appropriate measures" to respond to the test.
Susan Rice, the U.S. ambassador to the United Nations, said Washington and its allies intended to expand the sanctions put in place after Pyongyang's 2006 and 2009 nuclear tests.
This latest test was North Korea's third and most powerful nuclear test.
The official Korean Central News Agency (KCNA) said that the test on February 12 was to “protect our national security and sovereignty against the reckless hostility of the United States."
"It was confirmed that the nuclear test, which was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously, did not pose any negative impact on the surrounding ecological environment," the agency claimed.
In a later statement carried by KCNA, the North Korean Foreign Ministry warned that it will continue with unspecified "measures of greater intensity" if Washington maintains its hostility.
The move comes in defiance of UN Security Council measures calling on Pyongyang to halt nuclear-weapons development.
The test, which came ahead of U.S. President Barack Obama's State of the Union address later on February 12, was condemned by UN Secretary-General Ban Ki-moon, the five permanent members of the Security Council, and North Korea’s neighbors.
Obama called for "swift and credible action by the international community." He added that Washington would "continue to take steps necessary to defend” its allies in Asia.
Meanwhile, Washington acknowledged that Pyongyang had warned it before proceeding with its nuclear test. State Department spokeswoman Victoria Nuland said Washington had conveyed its opposition to such plans.
South Korea described the test as an "unacceptable threat to the security of the Korean peninsula and northeast Asia." Defense Minister Kim Kwan-jin indicated that his country's military had been placed on "high alert."
"South Korea will firmly cooperate with our allies, including the United States, and with the international community," he said. "We will make North Korea pay the price for its nuclear test."
The Foreign Ministry of China, North Korea's closest ally, said it has summoned the North Korean ambassador and protested strongly.
In South Africa, visiting Russian Foreign Minister Sergei Lavrov called on Pyongyang to abandon its nuclear-arms program and return to six-party talks involving the two Koreas, China, Japan, Russia, and the United States.
"[The North Korean nuclear test], of course, deserves condemnation," Lavrov said. "It shows once again that the [North Korean] authorities have ignored international law, neglected UN Security Council decisions, and it certainly deserves both condemnation and an adequate reaction."
Ban condemned the test as a "deeply destabilizing" provocation. Ban also expressed confidence that the Security Council "will remain united and take appropriate action."
UN sanctions were tightened against Pyongyang after a ballistic rocket launch in December.
Last month, the 15-nation Security Council passed a resolution threatening "significant action" against North Korea in the event of a new nuclear test or missile launch.
This nuclear test on February 12 was the first conducted under new leader Kim Jong Un, who succeeded his late father, Kim Jong Il, in December 2011.
The United States and North Korea's neighbors fear Pyongyang is seeking to produce a nuclear device that could arm a long-range missile.
With reporting by AP, AFP, Reuters, dpa