SEOUL » South Korea said today it is closely monitoring North Korean military sites because it believes the communist nation might fire more missiles, amid speculation that a long-range test launch aimed toward the United States is possible in coming days.

The North fired four short-range missiles off its east coast yesterday, just ahead of the Fourth of July holiday this weekend. The U.S. and Japan called the North's move “;provocative.”;

In 2006, North Korea launched its most advanced Taepodong-2 missile on July Fourth. The rocket broke apart shortly after takeoff.

“;We are keeping a close watch on North Korea as it may fire more missiles,”; said an official at South Korea's Joint Chiefs of Staff, adding the North's no-sail zone remains in effect in waters off its east coast through next Friday. The North announced last month a no-sail zone through that date for military drills.

The official, who spoke on condition of anonymity as he was not authorized to speak to media, did not elaborate on whether the North could fire short- and medium-range missiles or a long-range missile. Pyongyang is banned under U.N. resolutions from testing ballistic missiles.

In April, North Korea threatened to test-fire an intercontinental ballistic missile and warned of a nuclear test. The regime followed through with the underground atomic blast in May, leaving the ICBM test as its next likely step.

Speculation has centered on whether the North might try to fire a long-range missile toward Hawaii during the period up to next Friday, though officials and analysts are divided over the prospects.

Missile defenses were beefed up following a mid-June report in a Japanese newspaper that the North might fire a long-range missile toward the islands in early July.

Some, including Japanese Chief Cabinet Secretary Takeo Kawamura, believe a long-range missile launch could come this weekend. “;We cannot rule out the possibility,”; he said yesterday, citing Pyongyang's past behavior.

Several U.S. Defense Department officials, however, said there was nothing to indicate that North Korea is ready to launch a long-range ballistic missile, and there appears to be no immediate threat to the U.S.

Pentagon spokesman Bryan Whitman said the U.S. remains concerned about North Korea's missile and nuclear programs but called the North's launches of short-range missiles yesterday “;not unexpected.”;

South Korea's Yonhap news agency, citing an unnamed military official, reported that all four missiles flew about 60 miles.

Missile stock

A look at North Korea's missile arsenal:

Taepodong

This group of rockets is the pinnacle of North Korea's missile technology. Pyongyang claims they are vehicles to launch satellites as part of a peaceful space program. Satellite and missile technologies are interchangeable.

» Advanced Taepodong-2: under development. Potential range: about 5,000 miles, putting the U.S. West Coast, Hawaii, Australia and eastern Europe within striking distance.

» Taepodong-2: Three-stage rocket with a potential range of more than 4,100 miles, putting Alaska within striking distance. The first two stages are liquid-fueled, while the third is believed to be solid-fueled.

» Taepodong-1: Estimated range of 1,550 miles, twice as far as earlier Rodong missile, according to South Korea. Both lower stages are liquid-fueled, with a potential solid-fuel third stage. Accuracy is believed poor.

New Missile

North Korea has fielded a new intermediate-range ballistic missile, according to South Korea's Defense Ministry. With a range of 1,800 miles, it would almost reach Guam. Much of China and parts of Russia and the Philippines are also within striking range. North Korea reportedly used Russian SS-N-6 submarine-launched ballistic missile technology for the mobile, land-based missile. It reportedly is liquid-fueled with one or two stages.

Rodong

Japan is the likely target of this short-range missile. Rodong is almost identical to Iran's Shahab-3 and Pakistan's Ghauri II (Hatf V), the strongest evidence of the countries' collaboration. All three countries continue to refine the design. Estimated range of 620 to 930 miles. They are single-stage, liquid-fuel missiles on mobile launchers. Most have fairly poor accuracy, though some might have been fitted with more modern guidance systems.

Scud

Single-stage, liquid-fueled missile with a range of up to 500 miles. Known in North Korea by the name Hwasong, the SCUD B and SCUD C can reach only South Korea, but the SCUD D could target Japan. Accuracy is extremely poor.

Sources: STRATFOR global intelligence, South Korean Defense Ministry, Associated Press

Chronology of North Korea's missile program

Aug. 31, 1998: North Korea fires a suspected missile over Japan, calling it a satellite.

Sept. 13, 1999: North Korea pledges to freeze long-range missile tests.

March 10, 2003: North Korea fires a land-to-ship missile off the east coast into waters between the Korean peninsula and Japan.

October 2003: North Korea fires two land-to-ship missiles.

May 2005: North Korea fires a short-range missile into waters between the Korean peninsula and Japan.

March 8, 2006: North Korea fires two short-range missiles.

July 5, 2006: North Korea launches seven missiles into waters between the Korean peninsula and Japan.

July 15, 2006: The U.N. Security Council adopts Resolution 1695 demanding North Korea halt its missile program.

Oct. 9, 2006: North Korea conducts underground nuclear test blast after citing “;extreme threat of a nuclear war”; from the U.S.

Oct. 15, 2006: The U.N. Security Council adopts Resolution 1718 condemning the test, imposing sanctions and banning North Korea from activities related to its nuclear weapons program.

July 14, 2007: North Korea shuts down its main Yongbyon reactor, later starts disabling it.

Sept. 19, 2008: North Korea says it is restoring a key atomic reactor.

Oct. 11, 2008: U.S. removes North Korea from a list of state sponsors of terrorism.

April 5, 2009: North Korea launches a long-range rocket.

April 13: The U.N. Security Council adopts a presidential statement condemning North Korea's rocket launch.

April 14: North Korea quits six-party nuclear talks and vows to restart its nuclear facilities in protest against the U.N. statement.

May 25: North Korea conducts its second nuclear test.

June 12: The U.N. Security Council adopts Resolution 1874 sanctioning North Korea for its nuclear test.

June 18: A Japanese newspaper reports North Korea might fire a long-range missile toward Hawaii in early July. U.S. Defense Secretary Robert Gates says he has positioned more missile defenses around Hawaii.

Mid-June: North Korea announces a no-sail zone off its east coast from June 25 to July 10 for military drills.

Yesterday: North Korea test-fires four short-range missiles.

Sources: South Korean Defense and Foreign ministries and presidential office, Associated Press