The threat of terrorism against Americans and U.S. installations spread to Asia Friday as the State Department said Japan and South Korea may be potential targets for attack.

There are about 48,000 U.S. troops in Japan and 37,000 in South Korea, all of whom are covered by the warning. A worldwide caution issued in June will be expanded to include Asia, the department spokesman Richard Boucher said.

The U.S. Embassies in Tokyo and Seoul were open on Friday, will be closed as customary over the weekend and are due to reopen on Monday, he said. Boucher gave no details about the origin of the threat or how it was detected.

In Tokyo, the embassy issued a statement to all Americans living in the country, said it had unconfirmed information that terrorists may strike U.S. military facilities or places frequented by U.S. military personnel.

Embassy spokesman Patrick Linehan described the threat as "credible" but did not provide details such as when or where a possible attack might occur. He also did not say when the warning would be lifted.

Terrorist warnings are rare in Japan, Linehan said.

About 120,000 Americans live in Japan, according to the embassy including active members of the U.S. military.