Former U.S. President Bill Clinton arrived in North Korea on Tuesday to try to negotiate the release of two U.S. journalists convicted by the communist state of grave crimes, South Korea's Yonhap news agency said.

U.S. officials fear that North Korea might try to use the journalists as leverage in trying to resist international pressure to halt its arms program and weather U.N. sanctions over its missile and nuclear tests.

Following is a look at the North's missile program:

THE ARSENAL - North Korea has well over 1,000 missiles of various ranges. It has sold missiles and technology overseas, with Iran being one of the large buyers. The North may have about 1,000 ballistic missiles alone -- including as many as 685 Scud missiles of various types and 320 Rodong missiles. A U.N. resolution bans it from launching ballistic missiles.

SHORT-RANGE - North Korea has hundreds of non-ballistic missiles with ranges of 150 km (95 miles) or less. These include surface-to-air and surface-to-ship missiles. It fired a barrage of these missiles in May. There are no international agreements that bar it from test-launching these missiles.

SCUD TYPE - These include the Hwasong-5, with a range of about 300 km and the Hwasong-6, with a range of about 500 km. It first test-fired a Scud-B type missile in 1984. The North has steadily improved the ability of its Scuds to carry heavier warheads.

RODONG TYPE - The mid-range Rodong, first test-fired in 1993 and operationally deployed in 1998, has an estimated range of 1,000-1,400 km. It can hit all of South Korea and most of Japan.

IRBM - North Korea recently deployed a new type of mid-range ballistic missile some call the Musudan that can travel about 3,000 km. The intermediate range ballistic missile (IRBM) has a range that puts U.S. military bases in Guam at risk.

TAEPODONG TYPE - The Taepodong-1 is a multi-stage missile with an estimated range of 2,000-2,500 km. It uses liquid fuel and was fired over Japan in 1998.

The Taepodong-2 was first test-launched in July 2006 and flew for about 40 seconds before it blew apart. It is a multi-stage missile with a possible range of 6,700 km. Another version was launched in April and flew about 3,000 km before splashing into the sea. Analysts said that test showed the North had extended the range of its missiles but was still years away from building one that could strike the United States with a warhead.

The Taepodong-X is a solid fuel missile under development with an estimated range of 2,500-4,000 km.

North Korea does not have an operational missile that can hit the continental United States, analysts said.

ACCURACY AND PAYLOAD - Most analysts agree North Korea is some time away from building a nuclear warhead small enough to fit on a missile. The accuracy of the mid- to long-range missiles is also suspect. North Korea designed its ballistic missiles to carry conventional weapons, chemical weapons and eventually nuclear warheads.

(Sources: Center for Nonproliferation Studies, South Korean Defense Ministry, International Crisis Group)

(Reporting by Jon Herskovitz and Jack Kim)

Japan South Korea