Trade between the countries is minimal. U.S. companies can export to North Korea, but have to apply for a license to do so.
For nearly all items, other than food and medicine, there is a presumption the request will be denied, according to a 2011 report by the Congressional Research Service. There are tight controls on access to computers and software. Arms transfers and sales are forbidden.
The Treasury Department must approve any imports from North Korea, and weighs all requests on proliferation concerns and on questions of who might profit. Transfers from the North Korean government itself are generally prohibited. Using a North Korea-flagged vessel for any transaction is prohibited.
The U.S. blocks North Korea from receiving support from international financial institutions like the World Bank and International Monetary Fund. Aid from the U.S. itself is also heavily restricted. North Korea is barred from government programs offering credit and investment guarantees.
The U.S. blocks assets and business transactions with individuals and entities on a sanctions list maintained by the Treasury Department's Office of Foreign Assets Control because of their suspected involvement with North Korea in weapons proliferation and other arms-related material, money laundering, counterfeiting, narcotics trafficking and trade in luxury goods.
Klingner called for Washington to set up more unilateral sanctions against not only North Korean entities but also "all those other banks, businesses and government agencies of other countries that have facilitated North Korea's behaviors. Washington needs to call upon other nations to match our efforts."
Seoul slapped independent sanctions on rival Pyongyang after the 2010 sinking of a South Korean warship that Seoul blames on Pyongyang. North Korea denies involvement in the sinking, which killed 46 South Koreans.
Those measures banned North Korean ships from entering South Korean waters, slashed cross-border trade, restricted South Koreans from traveling to North Korea, and prohibited additional investment in North Korea. Humanitarian aid was drastically cut, except for certain cases, including that for infants and other vulnerable people. Travel restrictions have been eased in some cases.
South Korea also bans luxury items from being shipped into North Korea. That includes liquor, cosmetics, leather goods, fur products, carpets, jewelry, electronics, vehicles, boats, optical equipment, watches, musical instruments and antiques.
Seoul says it is open to all possibilities but isn't currently mulling new unilateral sanctions and believes the 2010 penalties are putting pressure on North Korea.
Japan bans exchanges of people, goods, ships and flights between the two countries. In principle, no North Korean citizens are allowed to visit, while Japanese are asked to abstain from traveling to North Korea.
Tokyo also limits the amount of cash that can be carried to North Korea to 100,000 yen, while payments of 3 million yen or more must be reported to the government. Japan also bars North Koreans from receiving technical education and training related to sensitive nuclear technology.
A government official, who requested not to be identified because the information was not given through formal disclosure procedures, said Tokyo will follow the lead of other major nations in determining any further sanctions.
Associated Press writers Sam Kim and Youkyung Lee in Seoul, South Korea, Elaine Kurtenbach and Malcolm Foster in Tokyo, Matthew Pennington in Washington, and Christopher Bodeen in Beijing contributed to this report.