Unidentified hackers breached travel records at the US Department of Defense, the US military said on Friday.

The department uncovered the breach on October 4, but a defense official told the Associated Press news agency that it may have occurred months earlier. The perpetrators were able to gain access to the personal and credit card data of up to 30,000 workers, he added.

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"The department is continuing to assess the risk of harm and will ensure notifications are made to affected personnel," the department said in a statement. It added that it was investigating the scale of the breach and the identity of the hackers.

The announcement came days after a US government report criticized the Department of Defense for slow progress in securing major US weapons systems from hackers.

Cybersecurity has become a high priority in the US military amid growing concerns about large-scale hacks from China and Russia. The department has said hackers attempt to breach its systems thousands of times a day.

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amp/aw (AP, Reuters)