SEOUL, South Korea - North Korea test-fired four short-range missiles today, South Korea's Defense Ministry said, a move that aggravates already high tensions following Pyongyang's recent nuclear test and U.N. sanctions imposed as punishment.

Two ground-to-ship missiles were fired from the eastern coastal city of Wonsan this afternoon, a ministry official said on condition of anonymity citing department policy.

The North also fired a third missile later from the east coast, but the exact site and the type of a rocket was not immediately known, the official said. Another ministry official - also speaking on condition of anonymity citing department policy - said the North later fired a fourth missile, though she provided no details.

Yonhap news agency, citing an unnamed military official, reported all four missiles flew about 60 miles  and identified them as KN-01 missiles with a range of up to 100 miles.

North Korea had earlier called for a no-sail zone in waters off its east coast through July 10 for military drills. That designation was viewed as a prelude to such missile tests.

The launches came as North Korea's relations with the United States, South Korea and other countries were already severely strained after its May 25 underground nuclear test and a series of missile firings. The U.N. Security Council adopted a tough sanctions resolution last month to punish the communist regime.

"We had expected that they will fire short-range missiles at any time," South Korea's Foreign Minister Yu Myung-hwan told The Associated Press at a reception held at the U.S. ambassador's residence to mark Independence Day on July 4, which falls this weekend. "It's not a good sign because they are demonstrating their military power."

In Tokyo, Japanese Prime Minister Taro Aso called the missile launches "provocative acts" and urged the North to refrain.

Russian Foreign Ministry spokesman Andrei Nesterenko also expressed concern over the North's missile launches, saying, "We have always asked North Koreans to abstain from actions that may exacerbate the situation."

The first launch came just before the U.S. ambassador's reception started in the late afternoon, while the second and third came as it was under way, based on times provided by the Defense Ministry. The timing for the fourth launch was not immediately available.

While it was not clear if the firings were meant to coincide with the event, the North did launch a long-range missile in 2006 in the early morning hours of July 5, which coincided with the July 4 holiday in the United States.

The United States is seeking Chinese support to enforce U.N. sanctions imposed on the North to punish it over the nuclear test. Philip Goldberg, in charge of coordinating the implementation of sanctions against the North, told reporters in Beijing that he had "very good conversations" with Chinese officials today, though not give details of the talks .

Separately, China's top nuclear envoy, Wu Dawei, left today for Russia as part of diplomatic efforts to push North Korea back to the stalled nuclear disarmament talks, according to the Chinese Foreign Ministry.

The trip will also take Wu to the U.S., Japan, and South Korea, the ministry said. The five nations have engaged in the talks since 2003 in an effort to persuade the North abandon its nuclear programs in return for economic aid and other concessions.

"The purpose of Wu Dawei's visit is to exchange views with relevant parties on the nuclear issues on the Korean peninsula," Chinese Foreign Ministry spokesman Qin Gang said in a regular press briefing.

Earlier in the day, Seoul's JoongAng Ilbo newspaper reported that North Korea could fire a barrage of missiles in coming days, including ballistic Scud or Rodong rockets that the North is banned from testing under U.N. resolutions.

North Korea has also threatened to test-fire an intercontinental ballistic missile. Last month, a Japanese newspaper reported that the North could test-fire a long-range missile toward Hawaii as early as July 4. The U.S. has increased defenses around the island state.

But Seoul's YTN television news network said Thursday that there are no signs of an imminent long-range missile launch.

The reported missile moves came after a North Korean ship - suspected of possibly carrying illicit weapons - changed course and was heading back the way it came after remaining under U.S. surveillance for more than a week.

The North Korean ship is the first vessel monitored under the new U.N. sanctions that seek to clamp down on Pyongyang'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.

The North has said it would consider the interception of its ships a declaration of war.

Associated Press writers Kwang-tae Kim, Hyung-jin Kim and Kelly Olsen in Seoul, Alexa Olesen in Beijing, Mari Yamaguchi in Tokyo and Mike Eckel in Moscow contributed to this report.