The Defense Agency has reinforced security at Self-Defense Forces facilities nationwide, especially those used jointly with U.S. forces, agency officials said Wednesday.

U.S. military bases worldwide were placed on "level D" alert, the highest of five categories, after terrorist attacks in the United States at the World Trade Center in New York and the Pentagon in Washington.

"As there is the possibility of terrorist attacks occurring in Japan, I have instructed the SDF to remain on high-alert mode," said Defense Agency Director General Gen Nakatani, who hastily returned to Japan from Jakarta on Wednesday morning after hearing of the attacks.