The U.S. defense secretary says he suspects North Korean succession moves may have prompted the sinking of a South Korean warship, and that he fears there may be more "provocations" to come.

Secretary Robert Gates spoke in San Francisco Thursday night about the widely held belief that North Korean leader Kim Jong Il is grooming his youngest son, Kim Jong Un, to take his place.

But, Gates said in a speech, he suspects the younger Kim "may have to earn his stripes with the North Korean military."

Gates also said he worries that "this might not be the only provocation from the North Koreans."

An international investigation concluded that the Cheonan was sunk in March by a torpedo from a North Korean submarine. Pyongyang denies any involvement in the sinking, which killed 46 sailors.

The United States responded to the sinking by joining South Korea in a series of joint naval maneuvers that have angered China. The first took place last month in the Sea of Japan involving the giant aircraft carrier USS George Washington and advanced jet fighters.

The George Washington is to participate in a future exercise in the Yellow Sea between South Korea and China.  A senior Chinese military official was quoted Friday as saying the exercise constitutes "a fresh provocation" to Beijing.

Some information for this report was provided by Reuters.