The United States said on Tuesday it had cracked down on companies involved in North Korea's suspected missile proliferation and in purchases of equipment that could be used in a nuclear weapons program.

The Treasury and State Departments said they had targeted Iran's Hong Kong Electronics and North Korea's Namchongang Trading Corporation under an executive order that would freeze their U.S. assets and bar U.S. firms from dealing with them.

The move appeared aimed at isolating the companies from the U.S. financial and commercial systems and, by extension, from other countries' banks and corporations who may resist doing business with them out of fear of falling afoul of U.S. laws.

It was immediately not clear whether either company actually has any U.S. assets that could be frozen.

The steps are part of an effort to get tough with North Korea, which conducted its second nuclear test this year and which has ceased carrying out a 2005 agreement to abandon its nuclear programs in exchange for economic and diplomatic benefits.

The U.S. Treasury said that it had targeted Hong Kong Electronics, which is located in Kish Island, Iran, because it had transferred millions of dollars of proliferation-related funds to North Korea from Iran.

"North Korea uses front companies like Hong Kong Electronics and a range of other deceptive practices to obscure the true nature of its financial dealings, making it nearly impossible for responsible banks and governments to distinguish legitimate from illegitimate North Korean transactions," Stuart Levey, undersecretary of the treasury for terrorism and financial intelligence, said in the statement.

The State Department described Namchongang as a Pyongyang-based "nuclear-related company" and said it has been involved in the purchase of aluminum tubes and other equipment "specifically suitable for a uranium enrichment program since the late 1990s."

Uranium enrichment is a process that can produce fuel for nuclear power plants or for nuclear weapons.