Six euro zone banks failed to meet the European Central Bank's capital requirements and were ordered to strengthen their balance sheets or tighten controls.

The ECB's annual review of banks shows that many lenders are struggling to make money in the face of ultra-low interest rates, high costs associated with their branches, and a string of money-laundering scandals.

The euro zone's top banking supervisors kept both their mandatory capital requirements and their" guidance", which is not mandatory, unchanged from the previous year, at an average of 2.1% and 1.5%, respectively.

However, six out of the 109 banks did not meet the capital recommendations, compared with one firm last year, and they will have to raise their tier 1 capital ratio (CET1) if they want to avoid new restrictions from the Supervisory authority.