Story highlights Sigal Mandelker of the US Treasury Department called for tougher rules in Hong Kong

A CNN investigation last year found multiple firms in Hong Kong with connections to North Korea

Hong Kong (CNN) Hong Kong needs to do more to combat North Korean smuggling and money laundering in the city, a top US official has warned.

The Asian financial hub has been linked to Pyongyang's efforts to violate sanctions. In December, South Korea seized a Hong Kong-flagged vessel it claims illicitly transferred oil to a North Korean ship, and a September United Nations report singled out shell companies with ties to Pyongyang.

"This shouldn't be a place where companies can establish themselves to help in the smuggling, ship-to-ship transfers, et cetera," Sigal Mandelker, the US Treasury's under secretary for terrorism and financial intelligence, told reporters Wednesday. CNN was not present at the briefing, but the US Treasury Department confirmed her comments.

Mandelker said she relayed her concerns in meetings with city officials.

"We have also stressed here in Hong Kong the importance of having the appropriate mechanism in place to enforce UN Security Council resolutions and other regulations prohibiting activities that facilitate financial transactions with North Korea," she said.

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