North Korea appears to be capable of fitting a nuclear warhead on some of its existing short- and medium-range missiles, spawning concerns about advances in the country's missile development, the head of a state-run think tank said Monday.

Washington assesses that the North may succeed in developing an intercontinental ballistic missile that can fly as far as the U.S. mainland by 2020, according to Choi Jin-wook, president at the Korea Institute for National Unification.

"From the technical perspective, North Korea is seen as being able to mount a nuclear warhead on its short-range Scud and medium-range Rodong missiles," Choi said in an interview with Yonhap News Agency.

North Korea fired off a Musudan missile, which has a medium range, on Friday, the birthday of the country's late founder Kim Il-sung, although the launch ended in failure.

If confirmed, it was the North's first test-launch of the Musudan ballistic missile, which is believed to be capable of flying as far as Guam, where U.S. military assets are stationed. The intermediate range missile is estimated to have a range of 3,000-4,000 kilometers.

There is high speculation that North Korea may carry out its fifth nuclear test or fire off a long-range missile ahead of the upcoming party congress slated for early May.

North Korea has ratcheted up its bellicose rhetoric since early March by claiming that it has succeeded in miniaturizing nuclear bombs and acquiring the technology that allows a ballistic missile to re-enter the atmosphere.

Early last month, the U.N. Security Council slapped tougher sanctions on Pyongyang for its nuclear test in January and long-range rocket launch in February.

He said that recent mass defections by North Koreans reflect that the sanctions are having an impact on curbing the North's behavior.

A group of 13 North Koreans who worked at a North Korea-run restaurant in China defected to South Korea earlier in the month amid the toughened international sanctions.

Restaurants in foreign nations serve as one of the main sources of hard currency for North Korea as dollar earnings are suspected of bankrolling the North's nuke and missile programs.

"More cases of such defections are likely to continue. I think China might have condoned the flight of the restaurant workers from the nation," Choi said. "This is an incredible change, compared with the past."

The chief predicted that North Korea may launch a charm offensive after the May party congress, but it would not seek sincere and meaningful talks on denuclearization.

North Korea's ruling party plans to hold its first congress in more than three decades in early May, when North Korean leader Kim Jong-un is likely to unveil new directions of policies.

"On the occasion of the party event, North Korea may attempt to seek talks with the U.S., but it is likely to launch nuclear provocations if its move ends in failure," Choi added. (Yonhap)