(Reuters) - Republican presidential hopeful Jeb Bush on Friday stepped up his criticism of Hillary Clinton, calling it "baffling" that she stored official U.S. State Department emails on a personal server rather than safer government systems.

"It's a dangerous world, and security would mean that you couldn't have a private server," the former Florida governor said in an interview with Radio Iowa on Friday morning. "It's a little baffling, to be honest with you."

Bush and other Republicans have pounded Clinton, the early front-runner for the 2016 Democratic presidential nomination, for using a personal email account for work during her four years as secretary of state from 2009 to 2013.

They claim she was trying to avoid transparency and could have posed a security threat.

Clinton has turned over to the State Department 55,000 pages of her email records that were stored on a private server, and asked the agency to release them. But officials said the review could take months, ensuring the controversy extends beyond the expected launch of her campaign.

Bush also used a personal email account during his time as governor but has released hundreds of thousands of the emails from that account. Bush told Radio Iowa he would not use a private email account if elected president.

White House spokesman Josh Earnest has said official policy requires that any work-related emails sent on personal accounts be documented on government systems.

Valerie Jarrett, a top adviser to President Barack Obama, said he had a very firm policy requiring that administration officials keep work email on government systems.

"He believes in transparency," Jarrett said in an interview on Bloomberg TV.

She would not speculate about whether Clinton broke those rules.

Clinton's team says previous secretaries of state also used private email addresses, and she quickly complied with requests to turn over the documents.

Congressional Democrats said Clinton has also cooperated with a U.S. House of Representatives panel probing the 2012 attack on a diplomatic facility in Benghazi, Libya. The panel's chairman, Republican Trey Gowdy of South Carolina, sent subpoenas demanding Clinton's emails relating to the incident.

The committee's senior Democrat, Representative Elijah Cummings, asked Gowdy to withdraw the subpoenas and publish hundreds of pages of Clinton emails already in the panel's possession.

A Gowdy spokesman said the panel would move forward with the subpoenas and would not release any emails until it had all relevant communications.