Updated

South Korea said Friday it is closely monitoring North Korean military sites because the communist nation may fire more missiles, amid speculation that a long-range test launch aimed toward the U.S. is possible in coming days.

The North fired four short-range missiles off its east coast Thursday, just ahead of the U.S. Independence Day holiday that falls this weekend. The U.S. and Japan called the North's move "provocative."

In 2006, North Korea launched its most advanced Taepodong 2 missile while the U.S. celebrated July Fourth, though the rocket broke apart shortly after takeoff and fell into the ocean.

"We are keeping a close watch on North Korea as it may fire more missiles," said an official with South Korea's Joint Chiefs of Staff. He added that North's no-sail zone announced last month for military drills remains in effect in waters off its east coast through July 10.

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-, medium- or long-range missiles. Pyongyang is banned from testing ballistic missiles under U.N. resolutions.

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 may try to fire a long-range missile toward Hawaii during the period up to July 10, 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 Japan's Chief Cabinet Secretary Takeo Kawamura, believe a long-range missile launch could come this weekend. "We cannot rule out the possibility," he said Thursday, 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 United States.

Pentagon spokesman Bryan Whitman said the United States remains concerned about North Korea's missile and nuclear programs but called North Korea's launches Thursday of short-range missiles "not unexpected."

South Korea's Yonhap news agency, citing an unnamed military official, reported that all four missiles flew about 60 miles (100 kilometers).

Separately, a North Korean ship, which had changed course after being shadowed for more than a week by the U.S. Navy, passed through the Taiwan Strait on Thursday and was heading toward North Korea, South Korean news channel YTN reported, citing an unidentified diplomatic source.

The Kang Nam 1 — originally believed to be bound for Myanmar, possibly with illicit weapons on board — turned around at Myanmar's request and the Southeast Asian country appeared to have offered compensation to the North, YTN said.

A U.N. resolution passed after North Korea's May 25 nuclear test seeks to clamp down on the country's trading of banned arms and weapons-related material by requiring U.N. member states to request inspections of ships suspected of carrying prohibited cargo.

North Korea has said it would consider any interception of its ships a declaration of war.

President Barack Obama has vowed that the U.S. won't reward North Korea's bad behavior, and his administration has been pressing China — a key North Korean ally — to enforce the new U.N. sanctions against Pyongyang.

In an interview with The Associated Press on Thursday, Obama said he was trying to "keep a door open" for North Korea to return to international nuclear disarmament talks, but the country must abandon its nuclear weapons programs before it can join the world community.

He also said there could be more sanctions in store for the North for its May nuclear test, saying the implementation of U.N. sanctions is "going very well."