In a single-sentence statement on Tuesday afternoon, the Bank of China said it has “already issued a bank account closing notice to North Korea’s Foreign Trade Bank, and has ceased accepting funds transfer business related to this bank account.”

A spokeswoman for the bank declined to say whether money in the account would be frozen or returned to North Korea. The spokeswoman, who insisted that her name not be used in keeping with bank policy, said the account had been closed by the end of April.

The Bank of China was the overseas banking arm of China’s central bank until the 1980s and is still majority-owned by the Chinese government, playing an important role in diplomatic and financial policy.

A succession of United States administrations has pressed China for many years to rein in the nuclear program of North Korea, a client state that China has long supported financially and diplomatically as a bulwark against South Korea and the American military forces based there. China has been reluctant until now to join international sanctions against North Korea, but there have been growing signs of dissatisfaction among Chinese intellectuals and possibly Chinese officials about the extent to which the North has shrugged off warnings and pushed ahead with tests in recent months of nuclear weapons and long-range missiles.

The United States Treasury imposed sanctions in March on the North Korean Foreign Trade Bank after accusing it of involvement in nuclear proliferation. Tom Donilon, the White House national security adviser, called at the time for China to stop conducting “business as usual” with North Korea.

Mr. Cai said that the move by the Bank of China appeared to be “predominantly symbolic,” but later added, “It could have practical consequences, because North Korea is already under such heavy international sanctions, and China is such an important economic channel for it.

“If China narrows the door to North Korea, then its economic operations or financial flows could be affected,” he said. “But primarily this appears to be a way of China showing its views about their behavior, so that North Korea is more likely to rethink its actions.”

The United Nations commission intends to document abuses as a way of pressuring North Korea’s leaders to improve conditions for the country’s 24 million people. Officials have few expectations that the North will grant access to the commission members, but Mr. Darusman said in March that an international inquiry “affords a measure of protection, especially when coupled with the prospect of future criminal investigations and the deterrent effect such a prospect may have on individual perpetrators.”