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The Netherlands-based TNT Express is still experiencing widespread service delays following the attack, FedEx said in a regulatory filing on Monday.
FedEx said it was unable to estimate when services at the unit would be fully restored.
The company said it was evaluating the financial impact of the cyber attack, but it was likely to be "material".
Shares of FedEx fell as much as 2.7 per cent to $213.07 in early trading.
In June, a new cyber virus spread from Ukraine to wreak havoc around the globe, crippling thousands of computers.
FedEx said it has experienced loss of revenue due to decreased volumes at TNT Express, incremental costs from contingency plans and remediation of affected systems.