FRANKFURT, Sept 4 (Reuters) - Germany's cartel office on Monday said it had expanded an investigation into suspected violation of antitrust rules in the country's steel sector, with Salzgitter SZGG.DE confirming some of its units had been searched.

"I can confirm that the cartel office has carried out searches in the area of flat steel on Aug. 22, 23 and 24 at seven companies as well as three private homes," a spokesman for the cartel office said.

"There is suspicion of anticompetitive collusion between the companies," he added.

(Reporting by Christoph Steitz; Editing by Maria Sheahan) ((christoph.steitz@thomsonreuters.com; +49 69 7565 1269; Reuters Messaging: christoph.steitz.thomsonreuters.com@reuters.net))