South Korea criticized North Korea for what it called "provocative behavior" after Pyongyang fired 25 short-range rockets into the ocean.
Seoul officials said the rockets were fired Sunday from North Korea's eastern coast and landed about 70 kilometers away in the Sea of Japan.
Defense Ministry spokesman Kim Min-seok said Monday the move was likely in response to ongoing U.S.-South Korea military drills.
"North Korea launched 25 rockets over three rounds directed towards the east from its eastern coast. Our South Korean military is considering this to be North Korea's firing demonstration, as an armed protest responding to Korea-US joint military exercises, and is additionally analyzing its specific intention. North Korea must not carry on with its provocative behavior that leads to military tension and regional insecurity. Our army is keeping a close watch on the possibility of additional launches and is now on military readiness posture," said Kim.
U.S. State Department spokeswoman Jen Psaki also urged North Korea to "refrain from provocative actions that aggravate tensions."
The so-called Foal Eagle South Korean-U.S. military exercises started in February and will run until mid-April.
Pyongyang routinely condemns such war games as rehearsals for an invasion, while Seoul and Washington maintain that they are purely defensive.
Since the drills began, North Korea has held several rounds of short-range missile launches.
Unlike its long-range missile tests, North Korea is not banned from testing short-range missiles under U.N. sanctions.
But the tests still keep Seoul authorities on alert, since North Korea has in the past shelled South Korean targets during military drills.
There are also concerns that the North Korean rockets could interfere with civilian airplanes passing nearby.
Earlier this month, Seoul officials said one of the rockets crossed the trajectory of a Chinese airliner with 200 people on board.