"Hopefully, these operations will use this opportunity to incorporate needed improvements into their safety and health programs," Michael Davis, MSHA deputy assistant secretary for operations, said in a statement.

Nevada's Leeville gold mine in Carlin is an underground operation that employs 355 people.

Mary Korpi, director of external relations for Newmont North America, said the company already has taken steps to bolster safety and health programs. Some of the programs are specific to Leeville, and others have been put in place at all Newmont mines, she said.

"All of it is geared toward getting compliance where it needs to be," Korpi said from the company's Elko office. "In spite of the improvements we've made in the past six months, our intent is to step back and re-examine procedures to make sure we're doing everything possible to stay in compliance."

The 15 mining operations are supposed to craft plans for reducing violations and will be monitored by health and safety inspectors for 90 days.

If they improve, MSHA said, they will not be listed as having a pattern of violations, which leads to greater scrutiny and tougher disciplinary action.

Among other things, the designation allows MSHA to interrupt production by ordering workers to leave a mine until a serious violation is corrected.

The agency has issued similar warnings to more than 40 U.S. mining operations since mid-2007, including 16 in June. Those mines reduced their serious violation rates, MSHA said.

Among the other operations warned are two mines and a processing plant controlled by Richmond, Va.-based Massey Energy Co., the nation's fourth-largest coal producer by revenue.

Massey issued a statement saying that several of its operations have been warned in the past and that all were able to correct problems.

Hidden Splendor Resources Inc. has been cited hundreds of times a year for violations at the Horizon mine, 11 miles west of Helper, Utah, according to MSHA.

"We have had problems with the mine and are taking measures to improve things," said Dan Baker, chief executive of Salt Lake City-based America West Resources Inc., the parent company of Hidden Splendor.

The problems were largely solved Saturday when the company closed a section of the mine that had 18 cave-ins over the past two years, Baker said. He said nobody got hurt by the roof falls.

Horizon, Utah's smallest coal mine, also has reduced its injury rate, from double the national average in 2007 to half the industry rate, said Joe Fielder, manager of the mine.

Though Horizon has reduced the risks, it was warned by MSHA based on mining conditions over the past two years, officials said.

Five of the mines are in Kentucky, four in West Virginia and three in Virginia. The dirt processing plant is in California.

Review-Journal reporter Alan Maimon contributed to this report.