The bank responded to CNBC's inquiry regarding an attempted credit card hack this past weekend.
"Through the monitoring of our systems, we detected an unsuccessful login attempt by a malicious actor; we locked a select number of cardholder accounts and also asked those cardholders to change their passwords as a precaution," Citi spokesman Danielle Romero-Apsilos told CNBC.
"There was no impact to Commercial Card services, nor was any customer data compromised," Romero-Apsilos said.
Citi provides financial services for the Government Travel Charge Card, or GTCC, which is used by Department of Defense personnel to pay for authorized expenses when on official travel.
CitiManager is the online portal used by the Defense Department to view statements online, make payments and confirm account balances.
The Pentagon's Defense Travel Management Office oversees the processing of the GTCC.
The Defense Department did not immediately respond to multiple requests for comment.