The United States is actively investigating additional Chinese companies over dealings with North Korea after imposing the first-ever sanctions on a Chinese firm earlier this week in connection with Pyongyang's nuclear and missile programs, a senior State Department official said Wednesday.

"We're investigating. The Treasury and State are investigating a number of companies around the world," Daniel Fried, the department's coordinator for sanctions policy, said during a Senate hearing in response to a question whether additional Chinese firms are under investigation.

"There are no limits and there is no administration red line of exempt countries or companies. We go where the evidence takes us," he said before the Senate Foreign Relations Subcommittee on East Asia and the Pacific, led by Sen. Cory Gardner (R-CO).

Asked again if he meant more Chinese firms are under investigation, Fried said, "I wouldn't argue with you." Later in the hearing, Gardner asked again whether "we are actively investigating Chinese entities" and Fried said, "Yes."

On Monday, the Treasury Department blacklisted China's Dandong Hongxiang Industrial Development Co. Ltd., its owner and three other company officials in a landmark move representing the first-ever sanctions on a Chinese entity over Pyongyang's weapons programs.

In addition to the Treasury Department sanctions that froze US-based assets of the Chinese firm and the four officials, the Justice Department has also pressed criminal charges against them for "conspiring to evade US economic sanctions" and "conspiracy to launder money instruments."

The sanctions could have powerful impacts on the North as they could scare other Chinese firms away from dealings with the North for fear that they could also be blacklisted by the US Pyongyang has long used Chinese firms to skirt international sanctions.

Fried warned Chinese firms against dealings with the North.

"It will also be useful if Chinese banks and companies understood that increasingly dealing with North Korean companies, especially those that are sanctioned, is going to be risky, frankly not worth it," the official said.

"The best sanctions are those that do not have to be applied because the credible threat of sanctions acts as deterrent. The US government's actions earlier this week demonstrate that we are in earnest and I can assure you that we are," he said.

Fried stressed that his warning against Chinese firms to avoid sanctionable activity is "not a phrase."

"Our actions on Monday indicate that Chinese companies ... are not off limits. That news will spread around the Chinese community.

We can also use various means to get the word out to Chinese businesses and banks that we are serious," he said.

Fried also said the US is "looking hard and actively" at North Korea's coal trade, saying that coal exports, which amount to about $1 billion a year, are "the largest single generator of foreign currency for North Korean economy."

The official also strongly suggested that the US is investigating the North's state air carrier, Air Koryo.

"It is true that we and our allies have curtailed Air Koryo's activities and restricted its ability. Third governments have restricted its ability to land. I don't want to discuss in an open session particular investigations, but we are well aware of Air Koryo's role in the North Korean system," he said. (Yonhap)