Former President George H.W. Bush has been moved to intensive care at a Houston hospital.

The 88-year-old is the oldest living president and is listed in "guarded condition", and is battling a fever.

He's been in the hospital for over a month.

A spokesman for Bush says he's had a series of setbacks and is on a liquids-only diet.

Despite his condition, Bush is described as alert and talking with medical staff.

Doctors say they are cautiously optimistic.