TORONTO (REUTERS) - Bank of Montreal said on Monday (May 28) it was contacted by fraudsters on Sunday who claimed they were in possession of the personal and financial information of a limited number of the bank's customers.

The bank said it believed the attack originated from outside the country and was confident the exposures that led to the theft of customer data had been closed off. It said it was working with relevant authorities to assess the situation.

The bank, Canada's fourth biggest lender, did not say how many customers were affected or if customers had lost money in the attack. It did not respond to requests for further comment.

Bank of Montreal said it was contacting customers who may have been hacked and would "support and stand by them."

"Customers are recommended to monitor their accounts and notify BMO with any suspicious activity," it said in a statement.

Shares in BMO were down 0.4 per cent.

Canada's six biggest banks have been collaborating along with the Bank of Canada to enhance their defenses against cyber attacks. The Bank of Canada said earlier this month that some attacks would inevitably succeed but it has recovery mechanisms in place to limit the damage.

Cyber attacks are increasingly common. Last year, credit monitoring firm Equifax said information on about 146.6 million names, 146.6 million dates of birth, 145.5 million US social security numbers, 99 million addresses and 209,000 payment card numbers and expiration dates, were stolen in a cyber attack.