China's Big Four state-owned banks have stopped providing financial services to new North Korean clients, according to branch staff, amid U.S. concerns that Beijing has not been tough enough over Pyongyang's repeated nuclear tests.

Tensions between the United States and North Korea have ratcheted up after the sixth and most powerful nuclear test conducted by Pyongyang on Sept 3 prompted the United Nations Security Council to impose further sanctions on Tuesday.

Chinese banks have come under scrutiny for their role as a conduit for funds flowing to and from China's increasingly isolated neighbour.

China Construction Bank (CCB) has "completely prohibited business with North Korea", said a bank teller at a branch in the northeastern province of Liaoning. The ban started on Aug 28, the teller said.

Frustrated that China had not done more to rein in North Korea, the Trump administration was mulling new sanctions in July on small Chinese banks and other firms doing business with Pyongyang, two senior U.S. officials told Reuters.

A person answering the customer hotline at the world's largest lender, Industrial and Commercial Bank of China Ltd (ICBC), said the bank had stopped opening accounts for North Koreans and Iranians since July 16. The person did not explain why or answer further questions.

The measures taken by the largest Chinese banks began as early as the end of last year, when the Dandong city branch of China's most international lender, Bank of China Ltd (BoC), stopped allowing North Koreans to open individual or business accounts, said a BoC bank teller who declined to be identified.

Existing North Korean account holders could not deposit or remove money from their accounts, the BoC bank teller said.

At Agricultural Bank of China Ltd (AgBank), a teller at a branch in Dandong, a northeastern Chinese city that borders North Korea, said North Koreans could not open accounts. The teller did not provide further details.

Official representatives for BoC, ICBC, CCB and AgBank could not be reached for comment.

Banks in Dandong have been under the microscope as tensions have risen, given their proximity to North Korea.

In June, the United States accused the Bank of Dandong, a small lender, of laundering money for Pyongyang.

Attempts to slowly choke off the flow of funds to and from North Korea come after the United States sanctioned a Chinese industrial machinery wholesaler that it said was acting on behalf of a Pyongyang bank already sanctioned by the United Nations for supporting the proliferation of weapons of mass destruction.

The Chinese wholesaler was found to be operating through 25 accounts at banks in China.

Although measures are in place, some bankers questioned how well the rules would be enforced.

Chinese lenders have experienced high-profile failures to police money-laundering in recent years, with some facing allegations that bankers were complicit in the movement of illicit funds.

"Asking whether we will be able to enforce the new rules is the same question as asking how tight our know-your-client checks are," said a senior corporate banker at the Bank of China who declined to be identified because of the sensitivity of the matter.

"There will always be holes," she said.