Japan does not plan to withdraw or scale down its contingent of troops in southern Iraq after a rocket was fired into its base there, a top official said Monday.

An unexploded, unarmed rocket was found inside the Ground Self-Defense Force base Saturday, the first time that's happened since the troops arrived on a humanitarian mission. There were no injuries.

Chief Cabinet Secretary Hiroyuki Hosoda said Monday that the attack was probably meant as a threat to the troops, but the government needs to investigate.

"The fuse was missing and we need to look into the case to find out about the intention of the attack," he said.

Hosoda said the government has no immediate plans to change the deployment. Japan has about 500 troops outside the southern city of Samawah on a strictly noncombat mission. "We don't think that the situation there has changed significantly," he said.

In past months, mortars and other projectiles have been fired at the base, most recently in August.

Hosoda said that the government will closely monitor the security situation before making a decision but added, "We will not make a decision based solely on this incident."

Prime Minister Junichiro Koizumi has been eager to help the U.S.-led coalition and raise Japan's international profile by dispatching troops on their largest -- and most dangerous -- overseas deployment since World War II.