The number of miners killed on the job in the United States fell for a second straight year to 34, the fewest since officials began keeping records nearly a century ago.

That was down from the previous low of 52 in 2008.

U.S. Mine Safety and Health Administration documents show 18 of the deaths occurred in coal mines, down from 29 in 2008; and 16 were in gold, copper and other types of mines, down from 22 in 2008. Most involved aboveground truck accidents on mine property, though some of the deaths resulted from rock falls and being struck by machinery.

Obama administration mine safety czar Joe Main said the numbers are encouraging, but he won't be satisfied until no miners are killed on the job.

"I think that's accomplishable, if you look at where we came from, and where we've come to," Main said.

The latest statistics are vastly improved, he said, from a century ago when hundreds, sometimes thousands of miners were killed each year.

The deadliest year in recorded U.S. coal mining history was 1907, when 3,242 deaths were reported. That year, the nation's most deadly mine explosion killed 358 people near Monongah, W.Va.

Main credits the decrease in deaths over the past year to beefed-up enforcement and stricter regulations in the wake of a series of mining disasters over the past four years in Kentucky, Utah and West Virginia.

In 2006, 73 miners were killed, including 12 who died in a methane explosion at the Sago Mine in West Virginia and five who died in a similar explosion at the Darby Mine in Kentucky. In 2007, 67 miners died, including six who were killed in the collapse of the Crandall Canyon mine in Utah.

Coal states reacted by revamping their mine safety laws, and Congress toughened federal rules that that brought a variety of advances. Among the improvements are caches of oxygen stashed in underground mines in case miners are trapped, refuge chambers to provide shelter in emergencies, and a communications system to allow underground miners to talk with colleagues on the surface.

Steve Earle, United Mine Workers of America international vice president for the Midwest, said while those were important improvements, getting inspectors into the field is the key.

"I can say without reservation that the safest day coal miners have is when inspectors are in the mines," he said. "The more we can put our inspectors in the mines, the safer those mines will become and the closer we will come to zero fatalities."

Mine safety advocate Tony Oppegard, who has successfully lobbied to triple the number mine inspections conducted in Kentucky, said mining remains a dangerous occupation.