North Korea has test-fired four short-range missiles, the South Korean Yonhap news agency reports.
The move, which follows Pyongyang's recent nuclear test and subsequent UN sanctions, is likely to aggravate regional tensions.
The missiles were fired from North Korea's eastern coast on Thursday, South Korean defence ministry officials were quoted as saying.
Al Jazeera's Steve Chao, reporting from Seoul, the South Korean capital, said the launches involved short-range, surface-to-ship, modified Silkworm missiles.
He said the launches came as no surprise because North Korea had previously warned that it would conduct military drills on its eastern coast.
"Many people took this as a sign that they were about to test missiles," he said.
"Here in Seoul, the government was expecting this.
"Earlier in the day, South Korea's police administration heightened the alert level, they began showing military drills in case of war and they also began showing anti-terrorism drills - a sign of how high the tensions are here on the Korean peninsula."
North Korea had earlier issued a no-sail zone in waters off its east coast through July 10.
Our correspondent said the announcement has given the North several more days to potentially test more missiles.
The missile launches come a week after the US extended economic sanctions against North Korea for another year as tensions grew over the communist state's nuclear activities.
Barack Obama, the US president, moved to prolong restrictions on property dealings with the North that were due to expire on June 26.
Obama said he acted "because the existence and risk of the proliferation of weapons-usable fissile material on the Korean peninsula continue to pose an unusual and extraordinary threat to the national security and foreign policy of the United States".
On Thursday, the US denounced the missile launches as the latest in a series of "provocative" acts, the AFP news agency reported.
"The North Koreans said they were going to launch these missiles. I don't think that's surprising that they've launched these missiles," Robert Gibbs, the White House spokesman, said.
"I take the North Koreans at their word that they're going to continue their provocative actions."
Despite the US administration saying that it would welcome fresh talks with the North, relations between the two continue to deteriorate amid international condemnation and sanctions in response to its recent nuclear test, and defiant rhetoric from Pyongyang.
North Korea warned in June that it would increase its nuclear activities and could launch military action against the US and allies after the UN Security Council announced new sanctions over Pyongyang's May atomic test.
The UN resolution banned all weapons exports from North Korea and authorised member states to inspect sea, air and land cargo, requiring them to seize and destroy goods that violate the sanctions.
North Korea's nuclear test in May defied a previous UN Security Council resolution adopted after the North's first underground nuclear test in October 2006.