After more than a month in a Texas hospital battling bronchitis, George H.W. Bush, the former US president and ex-envoy to China, took a turn for the worse and is in intensive care with a "stubborn fever", a spokesman said on Wednesday.

"He's had a series of setbacks now that have landed him in guarded condition in the intensive care unit," where he was transferred on Sunday, said spokesman Jim McGrath.

"Early last week it was really looking good, but then it seemed a couple of dominoes started falling and it was taking us in the wrong direction."

Bush, 88, was admitted to Methodist Hospital in Houston on Nov 7 for bronchitis treatment and released on Nov 19. But he was readmitted four days later after his cough flared up again.

Doctors had initially hoped to have the elder statesman home for Christmas, but he was forced to spend the holiday in the hospital, where he was joined by his wife Barbara, son Neil and grandson Pierce.

"I wouldn't say he was able to celebrate it in the traditional sense," McGrath said. "His family was with him, and he's conscious and he's able to engage in humorous banter with his doctor."

McGrath later said doctors were "cautiously optimistic", but there was no talk yet of a discharge date.

Bush has lower-body parkinsonism, which causes a loss of balance, and he has used wheelchairs for more than a year, McGrath said in an e-mail on Wednesday.

Bush, a Republican, served just a single term in the White House from 1989 to 1993, despite sending US troops to victory in Iraq in the first Gulf War and expelling Saddam Hussein's forces from Kuwait.

He was seen as a moderate, low-key president compared to Ronald Reagan, who preceded him, and Bill Clinton, who defeated him in 1992, but respect for his hard-edged realism in foreign policy has grown since his time in office.

He suffered a loss of support among his own conservative base after being forced to abandon his campaign vow: "Read my lips. No new taxes."

The decorated World War II veteran served in a number of top government posts, including vice-president, director of the Central Intelligence Agency and US ambassador to the United Nations.

His son George W. Bush served two terms as US president and also went to war with Iraq, this time sending US-led troops to Baghdad to overthrow Saddam, whom he had wrongly accused of hoarding weapons of mass destruction.

AFP - AP