US Secretary of Defence Robert Gates has told reporters in Beijing that North Korea could develop an inter-continental ballistic missile within five years.
Mr Gates is in China for talks with President Hu Jintao.
He said North Korea is becoming a direct threat to the United States.
However, Mr Gates says he does not believe North Korea would amass large numbers of the missiles, saying it would be a limited capability.
The Yonhap news agency last year reported that North Korea's arsenal already includes intermediate-range missiles that can hit targets at up to 3000km away. These missiles could hit all of Japan and put U.S. military bases in Guam at risk.