The United States says South Korea has its full support in seeking wide-ranging sanctions against North Korea following the sinking of a warship in March.
The sanctions include freezing trade with the north.
Secretary of State Hillary Clinton says the US military has been told to coordinate with South Korea's military to help deter future aggression by North Korea.
International experts have found parts of a torpedo that sank the ship on 26 March had lettering matching a North Korean design; 46 sailors were killed.
The US military says it will carry out anti-submarine and other naval exercises with South Korea in the near future.