Qatar National Bank said on Sunday that its systems were "fully secure" following a cyber attack which potentially exposed the names and passwords of tens of thousands of customers.

In a statement released on its website, QNB also said that the hack would have "no financial impact" on its customers but admitted for the first time that clients in Qatar had been targeted.

"QNB Group’s Risk Team monitored abnormal activity in our system environment, this was immediately communicated to relevant authorities," read the statement.

"We also took immediate steps and our systems are fully secure and operational. The attack only targeted a portion of Qatar-based customers."

The data leak, which was reported late last month, was said to include details such as mobile phone numbers, credit card numbers and international bank transactions.

Media reports claimed that a 1.4GB trove of documents was leaked online anonymously.

Among the information reportedly leaked were the bank details of several journalists working for satellite broadcaster Al-Jazeera, which has its headquarters in Doha.

Another folder leaked apparently revealed details of the work of internal and overseas intelligence agencies and the country's ruling family.

But many Qatar-based customers were left in the dark as in its only previous statement, QNB said it refused to comment on social media reports after reports of the hack circulated online.

The bank's statement on Sunday said some of the information released online was culled from "non-QNB sources".

"While some of the data recently released in the public domain may be accurate, much of it was constructed and contains a mixture of information from the attack as well as other non-QNB sources, such as personal data from social media channels," it said.

"We believe the nature of this incident is fundamentally an attempted attack on QNB Group’s reputation and not specifically targeted at our customers," it added, stressing all customers accounts were "secure".

QNB is one of the largest banks in the Middle East, and last month reported a first quarter seven percent increase in profits to almost $800 million, despite the impact of low global oil prices.