UPDATE 1-U.S. to audit 1,000 firms in illegal worker probe

* U.S. immigration agency to review firms' hiring records

* Firms have "public safety, national security" ties (Adds comment from government official)

PHOENIX, Nov 20 (Reuters) - The U.S. government will audit hiring records of 1,000 firms in agriculture, healthcare and other areas to determine if they have illegal immigrants working for them, authorities said on Friday.

The Immigration and Customs Enforcement agency, or ICE, said businesses served with audit notices were selected for inspection because they have connections to "public safety and national security."

The agency declined to identify the businesses because of what it said was the "ongoing, law enforcement-sensitive nature" of the audits. The agency said they included agriculture, food and healthcare firms as well as contractors serving government facilities.

"We are trying to tell people this isn't a stunt, it's not a flash in the pan, we are serious and we are going to continue to audit to make sure people are complying with the law," ICE assistant secretary John Morton told Reuters in a telephone interview.

"We will help them do that wherever we can, and for those people that don't want to do it, we will either fine them civilly, or if they are knowingly violating the law ... prosecute them," he added.

Immigration policy is a divisive political issue in the United States where some 12 million illegal immigrants live and work in the shadows and where Hispanics, the largest immigrant group, are an increasingly weighty voting bloc.

Earlier this year, President Barack Obama's administration broke with the policy of his predecessor George W. Bush that targeted undocumented workers for deportation, and instead implemented a strategy to go after U.S. employers hiring illegal immigrants.