The statement, just three sentences long, did not provide further details, including how long he would be away. The statement also said his offices will remain open for constituent services.
"I don't know how long he'll be out of action," said Frank Watkins, Jackson's spokesman.
Jackson's congressional colleagues noted his absence on the House floor last week but did not know why he was gone, said U.S. Rep. Danny Davis, a Chicago Democrat.
Jackson first won office in 1995 and represents Illinois' 2nd District, which includes neighborhoods on Chicago's South Side and south suburbs.
He is under investigation by the House Ethics Committee, facing allegations that he was involved in discussions about raising money for then-Illinois Gov. Rod Blagojevich's campaign in exchange for an appointment to President Barack Obama's old U.S. Senate seat.
Jackson also allegedly directed a fundraiser to buy plane tickets for a woman described by an associate as Jackson's "social acquaintance." The issue came up on the campaign trail during Jackson's 2010 re-election and in the March primary, which the Democrat easily won. Jackson has since called it a personal matter that he and his wife have dealt with privately.
Jackson has not been charged. He has repeatedly denied wrongdoing and insists he will be vindicated.
His father, the Rev. Jesse Jackson, was out of the country Monday and declined to comment on the matter. Messages left for Jesse Jackson Jr.'s wife, Chicago Alderman Sandi Jackson, were not immediately returned.
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