North Korea has moved to "increase its nuclear arsenal" by removing fuel rods from a nuclear reactor at Yongbyon, according to an official statement.
North Korea says it has taken rods from its Yongbyon reactor
The rods can be treated to produce plutonium for use in nuclear weapons.
The statement comes amid an escalating stand-off between North Korea and the US, after reports that Pyongyang may be preparing a nuclear test.
"Provocative statements and actions by North Korea only further isolate it," said a White House spokesman.
The spokesman, Scott McClellan, said the US was still hoping Pyongyang would return to stalled international talks.
"That's where our focus is. That's where our focus remains," he told reporters.
South Korea expressed "serious concern" about the North Korean statement, while China urged restraint from all sides.
"We have been taking steps necessary to increase our nuclear arsenal for defence purposes," the North Korean Foreign Ministry said in a statement carried by the official KCNA news agency.
There has been mounting speculation about the Yongbyon reactor since South Korea reported last month that it appeared to have been shut down, presumably in order to remove spent fuel rods.
Recent satellite images are reported to also suggest possible preparations for a nuclear test in North Korea's remote north-east.
The row over North Korea's nuclear programme first flared between Pyongyang and the Bush administration in 2002, when North Korea allegedly admitted to having a secret uranium programme.
North Korea responded by expelling International Atomic Energy Agency (IAEA) monitors and restarting its plutonium reactor at Yongbyon - frozen under an agreement with the Clinton administration.
Five megawatt nuclear reactor, 100km north of Pyongyang
Frozen in 1994 under agreement with US
Restarted in 2002, after N Korea says deal was over
Reportedly closed down in April 2005
May 2005: N Korea says reactor's fuel rods extracted to produce more plutonium
Six-party international talks were set up to address the stand-off, but North Korea pulled out of them last June.
The US envoy to North Korea, Christopher Hill, went to Beijing recently to try and persuade China to use its influence over North Korea to draw it back to the talks.
The US and Japan have been hinting at more coercive measures in recent weeks, but China and South Korea oppose sanctions and say more diplomacy is needed.
The IAEA said at the weekend that North Korea already has enough weapons-grade plutonium for five or six nuclear weapons from its last harvest of spent fuel rods, begun in 2003.
Analysts estimate the latest extraction of rods could give North Korea at least another two atomic bombs, once the fuel has cooled down in about three months' time.