SEOUL, Jan. 6-- South Korea on Tuesday announced its first official assessment that long-range missiles of the Democratic People's Republic of Korea (DPRK) may reach the U.S. mainland.

The Defense Ministry said in its biennial Defense White Paper that the DPRK is estimated to have a"capability to threaten the U. S. mainland"after five rounds of test-firing of long-range missiles.

The assessment was based on the DPRK's successful launch of Unha-3 rocket, which is believed to have a range of about 8,000 km, into orbit in December 2012. The previous test-firing failed in April that year.

The DPRK's Taepodong-2 long-range missile has a range of about 10,000 km, according to the white paper.

The white paper said the DPRK has a"significant"level of miniaturizing nuclear weapons because eight years have passed since its first nuclear test in 2006. Pyongyang conducted nuclear tests in 2006, 2009 and 2013.

It said the DPRK is estimated to have some 40 kilograms of plutonium, produced from the 5-megawatt reactor at its main nuclear facility in Yongbyon north of Pyongyang. It was the same estimation as in 2012.