Barack Obama signed a new order expanding the list of North Korean entities with frozen US assets [EPA]

Barack Obama, the US president, has announced broader financial sanctions against North Korea as part of a punishment over the sinking of the South Korean warship earlier this year.

Obama on Monday signed an executive order that added one North Korean citizen and three firms to the list of frozen US assets, in addition to three citizens and five state entities under existing sanctions.

It covers trade of conventional arms or luxury products, counterfeit currency or engage in money laundering, drug smuggling or other "illicit" activity that supports the North Korean government or its leaders.

In the order posted on the US Treasury website, Obama cited the Cheonan's sinking as well as the nuclear and missile tests as evidence North Korea poses "an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States".

Stuart Levey, the Treasury under-secretary for terrorism and financial intelligence, said the new order would help go after businesses "that enrich the highest echelons of the North Korean government while the North Korean people suffer".

Illicit gains

Levey said "the destructive course the North Korean government is charting is facilitated by a lifeline of cash generated through a range of illicit activities" that the US is determined to counter.

The names of the North Korean citizens and entities that were sanctioned were posted on the Treasury website.

US-North Korea relations have deteriorated since Obama took office, with his aides deeply unhappy about Pyongyang's decision to conduct nuclear and missile tests last year.

US officials were also unhappy over the March 26 sinking of the South Korean corvette Cheonan that killed 46 sailors.

The United States, South Korea and others have blamed the maritime incident on North Korea, which denies responsibility.

It is unclear how the expanded sanctions might affect the odds of reviving multilateral talks on ending North Korea's nuclear programme.

Nuclear talks

Last week North Korea's state media said the country's number two leader had told Jimmy Carter, a former US president, that Pyongyang was committed to denuclearising the peninsula by resuming six-party talks.

Carter was in Pyongyang to secure the release of Aijalon Mahli Gomes, a US citizen who was arrested in January and sentenced to eight years of hard labour for entering North Korea illegally.

Alan Romberg, an analyst with the Henry L Stimson Center think tank, told Reuters Obama's moves were partly to punish the North for the Cheonan sinking and partly to pressure Pyongyang to engage seriously on denuclearisation.

"It is also part of a continuing effort to put pressure on the North so that when we get back to denuclearisation talks they will get serious."

Romberg added that the additional sanctions may give Washington more leverage in any talks.