"The congressman is receiving intensive medical treatment at a residential treatment facility for a mood disorder," Rick Bryant, Jackson's chief of staff, told the Chicago Tribune in a statement Wednesday. "He is responding positively to treatment."
Bryant said in an e-mail that "rumors about him being treated for alcohol or substance abuse are not true."
NBC News reported earlier Wednesday that "friends and colleagues" said Jackson was being treated for alcoholism at an Arizona facility.
The congressman's wife, Chicago Alderman Sandi Jackson, had sent a text to NBC's Chicago affiliate asserting her husband "is not in rehab."
"I'm hopeful that my husband's doctors will be able to release something soon," she told the Tribune. "I'm in constant talks with them about Jesse's condition and his medical prognosis going forward."
Bryant's statement said information about Jackson's treatment is protected by federal law and the attending physician's name and treatment center "will not be disclosed in order to protect his continuing privacy," the Tribune said.
The congressman's father, the Rev. Jesse Jackson, declined to discuss his son's situation when approached by reporters at the annual conference of Operation Rain PUSH, saying it was "inappropriate" because of the setting.
In an interview with Politico, he denied an unconfirmed report aired by a Chicago radio station that his son attempted suicide.
"No, that's not true," the elder Jackson said. "He's with his doctor and getting treatment, regaining his strength. That's all I really want to say at this point."
The younger Jackson has been on a leave of absence since June 10 and may not return until September, aides say.
Jackson's office said in a June 26 statement he was suffering from exhaustion and would take a "medical leave of absence" for treatment.
The statement didn't give a date for Jackson's return, but Democratic lawmakers and aides said they believe it could be several weeks or even months, Politico reported Tuesday.
"I don't think he's coming back until at least September, if he comes back at all," said a Democratic source with knowledge of the situation told the Washington publication. "I think it's all up in the air."
ABC News, citing a source it did not name, reported that Jackson, 47, "will likely not return to Congress until after Labor Day" after Congress' August recess.
Jackson is being pressured to provide information about his illness and political plans, Politico said. Sen. Dick Durbin and Rep. Louis Gutierrez, both D-Ill., have called on Jackson to explain his absence.
The House Ethics Committee is looking at Jackson for his role in the scandal that felled former Illinois Gov. Rod Blagojevich. The U.S. Justice Department indicted Raghuveer Nayak, an Illinois businessman and fundraiser with ties to Jackson, in June on federal fraud and tax charges.
Jackson has denied any wrongdoing.