North Korea has hundreds of ballistic missiles that can target its neighbors in Northeast Asia, but will need foreign technology to upgrade its arsenal if it plans to pose a direct threat the United States, U.S. researchers said Tuesday.

The latest findings come from a research program investigating what North Korea’s nuclear weapons capability will be by 2020.

North Korea has conducted several nuclear tests, setting the region on edge with no sign of negotiations restarting to coax the country into disarming.

For now, the emphasis is on sanctions and military preparedness. Defense Secretary Ash Carter visits Japan and South Korea this week amid speculation the U.S. wants to place a missile defense system in South Korea in case of a ballistic missile strike from the North. However, Seoul is reluctant about the alleged project as it would alienate China. The U.S. already has deployed anti-missile radar in Japan.

U.S. military officials have expressed concern over the North’s growing nuclear capabilities. Navy Adm. William Gortney, commander of U.S. Northern Command and the North American Aerospace Defense Command, told reporters Tuesday that the U.S. believes North Korea has the ability to miniaturize a warhead to put on an intercontinental ballistic missile.

U.S. officials are most concerned with a long-range missiles called the KN-08 that has been displayed in military parade. It is said to have the capability to be launched from a road-mobile vehicle and would therefore be difficult to track via satellite.

But the research published Tuesday by the North Korean Futures Project stresses that for now the principal threat from North Korean missiles is to its neighbors in Asia. The project is conducted by the U.S.-Korea Institute at John Hopkins School of Advanced International Studies and National Defense University's Center for the Study of Weapons of Mass Destruction.

Aerospace engineer John Schilling and a research associate at the institute, Henry Kan, say Pyongyang's current inventory of about 1,000 missiles, based on old Soviet technology, can already reach most targets in South Korea and Japan.

"North Korea has already achieved a level of delivery system development that will allow it to establish itself as a small nuclear power in the coming years," they write in a paper published on the institute's website, 38 North.

Despite North Korea’s recent success in launching a rocket into space, Pyongyang faces greater technical challenges in launching a missile across the Pacific at the U.S.

Analysis says the North may already be able to field a limited number of long-range Taepodong missiles in an emergency, but they would be unreliable, vulnerable to preemptive strikes and inaccurate.

The analysis says foreign assistance could be critical to overcome the technological and engineering hurdles North Korea faces in developing better missiles. Those hurdles include progress on high-performance engines, heat shields, guidance electronics and rocket motors that use solid fuel instead of liquid fuel, it says.

And that's become tougher as North Korea's international isolation has intensified since its first nuclear test in 2006.

That hasn't stopped its nuclear program. According to a recent estimate by the Washington-based Institute for Science and International Security, the North likely has enough fissile material for at least 10 weapons, and that could increase to between 20 and 100 weapons by 2020.

But whereas the basic designs and production infrastructure are largely in place for the nuclear program, technological progress  has been slower, the analysis says. North Korea has failed to make the kind of advances that Iran and Pakistan have made, although both countries relied on North Korean assistance for missiles in the 1990s.

Last October, the commander of U.S. forces in South Korea, Army Gen. Curtis Scaparrotti, said North Korea may be capable of fielding a nuclear-armed KN-08 missile that could reach U.S. soil, but because it has not tested such a weapon the odds of it being effective were "pretty darn low."

The Associated Press contributed to this report.