About 400 employees at Chesapeake Energy Corp. were laid off Tuesday as part of an ongoing effort to cut costs and focus the company's oil and natural gas production efforts, executives said.

Tuesday's cuts affect about 13 percent of the company and include about 330 employees in Oklahoma City.

"The decision to reduce head count did not come easily for the leadership team," CEO Doug Lawler said in an email to employees Tuesday morning. "Dedicated, value-driven, hardworking people have been affected. You have my personal assurance that we are treating these employees fairly, respectfully and with considerable effort to assist them with their personal and career transition."

The job cuts come after three years of changes at Chesapeake and throughout the industry as low commodity prices led companies to focus on fewer core assets. Chesapeake has sold about 25 percent of its wells during that time.