President George H.W. Bush and wife Barbara attend the premiere of HBO's new documentary on his life.(Charles Krupa/AP)

Former president George H.W. Bush remains in a Houston hospital with a rising fever, his office confirms. 

Spokesman Jim McGrath said in a statement Wednesday evening that doctors put Bush in the intensive care unit Sunday and on a liquid diet. As of Thursday morning, there was no update on Bush's condition, but doctors are cautiously optimistic. 

“Following a series of setbacks including a persistent fever, President Bush was admitted to the intensive care unit at Methodist Hospital on Sunday where he remains in guarded condition," McGrath said. "Doctors at Methodist continue to be cautiously optimistic about the current course of treatment. The President is alert and conversing with medical staff, and is surrounded by family.”

The 88-year-old has been in and out of Methodist Hospital since early November, battling a severe cough. In recent days, he has been undergoing physical therapy to rebuild his strength. Doctors expected he would be able to go home for Christmas, but Bush developed a fever that has left him weak.

On Christmas Day, the former president ate Chinese food with family members in the hospital.

Bush's fever is being treated with Tylenol, according to the AP, but doctors haven't figured out the cause. Meanwhile, the cough that brought him to the hospital has improved.

Bush held the presidency from 1989 to 1993. The oldest living president, he marked his 85th birthday in 2009 by skydiving. He has a form of Parkinson's disease and now uses a scooter to get around; his current illness is unrelated.