North Korea has fired several unidentified short-range projectiles from its eastern coast, the South Korean Joint Chiefs of Staff said Saturday. The initial U.S.assessment is that the projectiles were short-range missiles, CBS News national security correspondent David Martin reported.

South Korea's military bolstered its surveillance in case there are additional launches. The country initially reported Saturday that a single missile was fired, but later issued a statement saying "several projectiles" had been launched and that they flew up to 125 miles before splashing into the sea toward the northeast.

There is no doubt the launch was a message, although it's not clear who it was addressed to, according to CBS News' Elizabeth Palmer. The North Korean regime may be signaling its frustration with the U.S. over stalled negotiations to swap sanctions relief for denuclearization, after talks between President Trump and North Korean leader Kim Jong Un ended in a stalemate in February.

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Mr. Trump tweeted Saturday about Kim but didn't mention North Korea's latest action.

"Anything in this very interesting world is possible, but I believe that Kim Jong Un fully realizes the great economic potential of North Korea, & will do nothing to interfere or end it," he tweeted. "He also knows that I am with him & does not want to break his promise to me. Deal will happen!"

Anything in this very interesting world is possible, but I believe that Kim Jong Un fully realizes the great economic potential of North Korea, & will do nothing to interfere or end it. He also knows that I am with him & does not want to break his promise to me. Deal will happen! — Donald J. Trump (@realDonaldTrump) May 4, 2019

The launch could also be designed to show North Korea's anger with the South Koreans for participating in two weeks of joint military exercises with the U.S. Air Force.

South Korea said it is "very concerned" about the weapons launches, calling them a violation of last year's inter-Korean agreements to reduce animosities between the countries. It also urged North Korea to stop committing acts that would raise military tensions and join efforts to resume nuclear diplomacy.

Japanese Foreign Minister Taro Kono and U.S. Secretary of State Mike Pompeo held telephone talks after North Korea's launch, which the foreign minister confirmed didn't post a security risk to his country. But all countries in the region will be watching very closely to see if this is just the beginning of provocative actions by North Korea.