More Chinese companies and individuals are under investigation for suspected violations of sanctions on North Korea’s nuclear proliferation, a senior U.S. official indicated Wednesday. Days after Monday's announcement that four Chinese individuals and a Chinese company face criminal charges and sanctions for their alleged support of North Korea’s nuclear weapons program, the State Department’s coordinator for sanctions policy told U.S. lawmakers he would not argue with suggestions that the probe could widen. “It would 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 and frankly not worthy,” Daniel Fried told a Senate Foreign Relations Subcommittee on Wednesday, describing the problem as “the heart of the matter.” Dandong Hongxiang Industrial Developmental Company Limited, as well as its chairwoman Ma Xiaohong and three top executives, were charged Monday with conspiring to evade sanctions against North Korea, including by facilitating money laundering through U.S. financial institutions. Another senior State Department official said Washington is not “fully satisfied” with Beijing’s North Korea sanctions implementation. “There is much more that we believe China can and should do,” said Assistant Secretary of State Daniel Russel when testifying at the Senate subcommittee hearing. Russel added that President Barack Obama has raised with Chinese leaders practical ways that China can enhance the effectiveness of sanctions, including strengthening border controls and limiting access to North Korean banks. Pyongyang changes tactics North Korea for years has been under extremely tight international sanctions, which have largely halted North Korean trade with countries other than China. But new research says Pyongyang's state-run trading companies have adapted and are evading international sanctions by hiring more capable Chinese middlemen. These contacts help Pyongyang access Chinese supplies and foreign firms located in China, which has actually improved the North's ability to procure banned equipment according to researchers from Harvard University and Massachusetts Institute of Technology (M.I.T.) In a report titled “Stopping North Korea, Inc.: Sanctions Effectiveness and Unintended Consequences,” Harvard University faculty member John Park and M.I.T. researcher Jim Walsh said North Korean firms have increased use of embassies as vehicles for procurement of banned materials. Another tactic is expanding the use of Hong Kong and commercial hubs in Southeast Asia for procurement. In Beijing this week, China voiced strong opposition to Monday's sanctions, characterizing them as legal overreach by Washington. “If any country tries to exercise long-arm jurisdiction by enforcing its domestic laws over China's enterprises and individuals, we are firmly opposed to that,” said Chinese Ministry of Foreign Affairs spokesperson Geng Shuang on Tuesday. The Dandong-based Hongxiang Company was also under investigation by Chinese authorities for its connection with Kwangson Banking Corporation, a North Korean bank suspected of financing Pyongyang’s nuclear weapons program. Kwangson Banking Corporation was previously designated by the United States and United Nations for providing financial services in support of North Korea’s weapons proliferation.