The U.S. will freeze additional assets, prevent more individuals from traveling abroad and collaborate with banks to stop suspect transactions, they said.
The U.N. Security Council has imposed stiff sanctions on North Korea in recent years to punish the regime for defying the world body by testing nuclear weapons and long-range missiles, and illegally selling arms and weapons.
The council earlier this month approved a statement condemning the Cheonan incident, but did not directly blame Pyongyang. Still, the U.S. and South Korea are adamant that North Korea apologize for the incident or face punishment, and warned against further provocations.
An international team of investigators pinned the explosion of the Cheonan on a torpedo fired from a North Korean submarine. But North Korea denies any involvement and has threatened war if punished for it.