The US and South Korea have condemned North Korea's alleged move to "increase its nuclear arsenal".
North Korea says it has taken 8,000 rods from its Yongbyon reactor
Washington and Seoul were responding to Pyongyang's statement that it had successfully removed fuel rods from a nuclear reactor at Yongbyon.
The rods can be treated to produce plutonium for use in nuclear weapons.
A White House spokesman said the move could "only further isolate" North Korea, while South Korea expressed its "serious concern".
The North Korean announcement came amid heightened concerns, following reports that the Communist state may be preparing a nuclear test.
The International Atomic Energy Agency (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.
"We have been taking steps necessary to increase our nuclear arsenal for defence purposes," the North Korean Foreign Ministry said in the statement carried by the official KCNA news agency.
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
In response, White House 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.
The South Korean foreign ministry, meanwhile, called on Pyongyang to "immediately halt actions that have a negative impact".
"We strongly urge North Korea to return to the six-party talks without delay," the ministry said, according to South Korea's Yonhap news agency.
Japanese Prime Minister Junichiro Koizumi expressed similar views, while China urged restraint from all sides.
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 IAEA monitors and restarting its plutonium reactor at Yongbyon - frozen under an agreement with the Clinton administration.
In June the North pulled out of international talks involving the two Koreas, the US, China, Japan and Russia.
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.