Seoul, South Korea (CNN) -- North Korea has fired 25 short-range rockets from its east coast into open water, in what appears to be a "provocative" action, a South Korean Ministry of National Defense spokesman said Sunday.
"We evaluate it as a firing demonstration in response to the joint drill between South Korea and the U.S. We are currently additionally analyzing its intention," said spokesman Kim Min-Seok, warning, "North Korea should halt any actions that can stir military tension and create uneasiness to the neighboring countries."
South Korean officials said earlier that the North had fired only 10 rockets.
The rockets appear to be FROGs, which stands for "Free Rockets Over Ground." They were developed in the Soviet Union before the advent of missiles, Kim said.
"It does not have a guidance system and is (a) free-fall system. North Korea had developed it in the '60s," the spokesman said.
Earlier Sunday, South Korea said the rockets appeared to travel about 70 kilometers into the Sea of Japan, according to defense officials who asked not to be identified by name as a matter of security protocol.
Such actions have drawn criticism from South Korea and the West.
The U.S. State Department said it's aware of the reports of rocket launches by North Korea and is closely monitoring the situation.
"We once again call on North Korea to refrain from provocative actions that aggravate tensions," State Department spokeswoman Jen Psaki said.
But North Korea has defended the series of short-range launches it has carried out in recent weeks.
"It is justifiable self-defense behavior for us to conduct these military exercises in order to preserve peace in the region and to protect the safety of our people and our country," the government said, according to state-run media.