South Korean defense ministry said Tuesday that North Korea appeared to have achieved a “significant” level of technology to build mini nuclear devices that could be fitted on the tip of a missile. It gave the warning in a white paper, which is due to be released later Tuesday, adding that Pyongyang is also working on a highly enriched uranium program, according to Agence France-Presse (AFP).

South Korea said that the North most likely procured about 88 pounds of weapons-grade plutonium by reprocessing spent nuclear rods. North Korea has conducted three nuclear tests so far, with the most recent one in February 2013.

"North Korea's capabilities of miniaturizing nuclear weapons appear to have reached a significant level," the South Korean defense ministry said, according to AFP.

In 2007, North Korea had put on hold the five-megawatt reactor at its main Yongbyon nuclear complex. But, in 2013, Pyongyang started to renovate the reactor, which has been the country’s main source of weapons grade uranium, AFP reported. South Korean defense officials also reportedly said that the North may have the capability to develop missiles that could threaten the American mainland.

In November, North Korea had launched a submarine, which could fire ballistic missiles, sources told South Korean news agency Yonhap, adding that North Korea had "imported a Soviet-era Golf-class diesel submarine and modified it."

Victor Cha, chief Korea analyst at the Center for Strategic and International Studies, reportedly said that it is important for the allies to prepare for North Korean threats.

"The United States is likely to see the next series of North Korean nuclear and ballistic missile tests. These may demonstrate Pyongyang's crossing of a new technology threshold, such as warhead miniaturization, a uranium-based test, more accurate ballistic missile or nuclear fusion capabilities," Cha said, according to Yonhap, adding: "In any event, the administration must be prepared to meet these provocations with concrete measures that acknowledge the necessity of deterring a nuclear North Korea."