WASHINGTON (Reuters) - A top aide of Republican Senator Lamar Alexander was arrested on Wednesday in connection with child pornography allegations, the U.S. Justice Department said.

"Jesse Ryan Loskarn, 35, of Washington, D.C., was arrested this afternoon by the U.S. Postal Inspection Service based on probable cause for possession and distribution of child pornography charges," said Peter Carr, a Justice Department spokesman.

Loskarn, who has since been placed on unpaid leave as the Tennessee lawmaker's chief of staff, will appear at a hearing in federal court on Thursday.

"I am stunned, surprised and disappointed by what I have learned," Alexander said, adding that his office is "fully cooperating with the investigation."

Alexander said he had been told earlier in the day by the U.S. Senate's legal counsel that law enforcement agents were searching Loskarn's house.

The senator named longtime aide David Cleary to be his new chief of staff. Cleary has worked with Alexander since 2006 and has been Republican staff director for the Senate Health, Education, Labor and Pensions Committee since 2012. Alexander is the senior Republican on the panel.

Alexander is seeking a third term in the Senate in next year's congressional elections. While he is facing a challenge in the Republican primary, Alexander is thought to be in position to win re-election.

(Reporting by Susan Heavey, Richard Cowan and Deborah Charles; Editing by Jim Loney and Jackie Frank)