U.S. sanctions on a Chinese bank aiding nuclear-armed North Korea are "extremely long overdue" and the Trump administration should also consider action against even larger banks, according to a former U.S. intelligence official.

"This is a significant step but really a first step in what should be a much more extensive program against the Chinese entities that are violating U.S. law," said Bruce Klingner, former chief of the CIA's Korea branch and now senior research fellow for Northeast Asia at the Heritage Foundation's Asian Studies Center.

On Thursday, the Treasury Department designated China's Bank of Dandong to be a "primary money laundering concern." It said the small bank "acts as a conduit for illicit North Korean financial activity." Two Chinese individuals were also targeted in the government's action.

The U.S. Treasury Department said its FinCEN (or Financial Crimes Enforcement Network) is taking steps that will prevent the bank from continuing to be an avenue for North Korea to have access to U.S. and international financial systems. It said the bank's activities include "facilitating millions of dollars of transactions for companies" or front entities known to have ties to the hermit regime's nuclear weapons and ballistic missile programs.

"For whatever reason, the U.S. has not taken action against China the way we had against other nations for their involvement with Iran," said Klingner.

Klingner said it was time to step up the sanctions and consider "imposing significant fines against China's big-four banks if they are found to be complicit" in illicit dealings with North Korea. In the past, he noted that the U.S. had imposed billions of dollars in fines on major European banks for money laundering, including violating sanctions with Iran.

That said, the former U.S. intelligence official added that the large Chinese banks maybe "too big to designate as money laundering concerns because it would have an impact on the U.S. and world economy."

"The medium to small banks," Klingner said, "could be designated depending on the available evidence."

President Donald Trump appears to have waited to impose sanctions against Chinese banks because he believed Chinese President Xi Jinping would be able to help persuade North Korea's leader Kim Jong Un to abandon the regime's nuclear and missile programs. A frustrated Trump, though, tweeted June 20 that "it has not worked" and many took it to mean the China outreach on North Korea had failed.