Sears Holdings Corp said it was the victim of a cyberattack that likely resulted in the theft of some customer payment cards at its Kmart stores.

The U.S. Secret Service confirmed it was investigating the breach, said Reuters, which occurred in September and compromised the systems of Kmart, which has about 1,200 stores across the United States. The breach did not affect the Sears department store chain.

A Sears spokesman said he could not say how many credit and debit card numbers had been taken. He added that the personal information, debit card PIN numbers, email addresses and Social Security numbers of its customers remained safe, said Reuters.

Sears said that the attackers used malicious software that was undetectible using anti-virus software, highlighting the challenge of keeping up with the evolving techniques of computer hackers. Company spokesman Chris Brathwaite said Sears had been upgrading its systems even before the recent spate of incidents involving retailers, which included a massive breach of the systems of Target Corp in late 2013, Reuters said.

"Our IT team was able to quickly remove the malware and we are deploying further advanced software to protect our customers' information," Brathwaite said.

Kmart apologized to its customers last weekend and said it was working with federal authorities, banking partners and security firms in the probe.