Prime Minister Junichiro Koizumi said Sunday the Japan Coast Guard fired on an unidentified ship believed to be from North Korea in "legitimate self-defense."

Koizumi also expressed his "regret" that the ship, which sank Saturday night after exchanging fire with coast guard vessels, was not seized.

"There is a need to study the situation well," Koizumi told reporters at his official residence.

The prime minister said that his government will study legal measures to beef up Japan's ability to deal with such incidents.

The ship sank in the East China Sea on Saturday night following an exchange of fire with Japan Coast Guard vessels. About 15 crew members aboard the unidentified ship went missing, according to the coast guard.

Taku Yamasaki, secretary general of the dominant Liberal Democratic Party, separately said Sunday that the Japan Coast Guard's action was "inevitable."

"If neither the Japan Coast Guard nor the Defense Agency fail to respond to fleeing mystery ships, we would invite huge distrust from the Japanese people," he told reporters. "I would expect to get various opinions about the sinking of the ship, but the problem stems from its trying to flee. As a whole, it could not be helped."