Mizuho Bank on Monday fully restored its automated teller machine services a day after a system failure prevented customers from withdrawing cash from thousands of the major Japanese bank's machines across the country.

"We are deeply sorry for having caused great inconvenience to our customers," Mizuho Bank President and CEO Koji Fujiwara told a press conference, adding that its response after the defect was "not sufficient."

The problem that began on Sunday affected over 4,000 machines which stopped functioning at one point while 5,244 bank cards and books were stuck inside the machines and not returned to their owners, according to Mizuho Bank. All of its ATMs were back in operation by 3 p.m. Monday.

An out-of-service Mizuho Bank ATM is pictured in Tokyo on March 1, 2021. (Kyodo)

According to Mizuho Bank, its systems were overwhelmed when updating the status of fixed deposit accounts and processing other transactions.

Fujiwara pledged to get to the bottom of the issue and prevent any recurrence, acknowledging his management responsibility.

A senior Mizuho Bank official told reporters earlier in the day that the bank will conduct a third-party investigation.

As a result of the system failure, the Financial Services Agency has decided to order Mizuho to submit a detailed report on the incident including how it dealt with affected customers, according to sources close to the matter.

"It is important to thoroughly investigate the cause and prevent any recurrence" of such a system failure, Chief Cabinet Secretary Katsunobu Kato told a press conference.

Its internet banking service, which also failed to complete some transactions, has since been restored.

Mizuho said on its website that it is working on contacting customers whose cash cards went unreturned so it can return them. It also asked those who had to pay extra fees to withdraw money by using the ATMs of other banks to consult with its branches.

Photo taken on Feb. 28, 2021, shows a Mizuho Bank ATM in Tokyo hit by a glitch affecting cash withdrawals and other transactions at some of the major Japanese bank's outlets. (Kyodo)

Other banks are closely following the incident because Monday falls on the first day of March and there is usually a surge in transactions at the beginning of each month.

The banking arm of Mizuho Financial Group Inc. was informed of the problems with its ATMs on Sunday morning.

Mizuho customers expressed dismay at the problem as the bank has suffered similar system failures in the past.

Many complained about how their cash cards were "sucked into" the machines, as well as how their calls were going unanswered on Sunday.

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While some customers said they can no longer trust the bank, one customer tweeted about having to wait by an ATM for four hours.

In April 2002, when its affiliated banks were reorganized, system problems resulted in disruptions of about 2.5 million transactions.

In March 2011, ATM and counter transactions were temporarily suspended and more than 1 million money transfers were delayed soon after the massive earthquake and tsunami in northeastern Japan.