South Korea's defense ministry said North Korea appears to have achieved a "significant" level of technology to miniaturize a nuclear device that could fit on the tip of a missile.
The ministry announced the news in a white paper released Tuesday. It said North Korea has not yet demonstrated its miniaturization capacity.
The ministry said Pyongyang has obtained some 40 kilograms of weapons-grade plutonium by reprocessing spent nuclear fuel rods. Seoul said North Korea is working on a highly enriched uranium program.
The North is believed to have the capacity to develop missiles that could reach the mainland of the United States. It has conducted a series of long-range missile tests.
South Korea's Yonhap news agency quoted an unnamed defense ministry official as saying the North is believed to have increased the range of one of its missiles from 7,000 kilometers to 10,000, but it said there is no evidence that Pyongyang has put such missiles into service.
On Sunday, North Korea lashed out at the United States for imposing new sanctions as punishment for the alleged hacking of computers at Sony Pictures. The state-run Korean Central News Agency called the new sanctions "hostile and repressive" and said they will only harden Pyongyang's resolution to defend the sovereignty of the country.
President Barack Obama announced the new sanctions on Friday. They target three organizations and 10 individuals. White House officials said the sanctions are just the first step in seeking to punish North Korea for numerous provocations. The U.S. already had sanctions in place against North Korea for its nuclear program, but the new sanctions are the first imposed related to cyber-security.