WASHINGTON (AFP) - Former United States president George H.W. Bush is spent a fourth night at a Texas hospital on Friday after experiencing breathing problems, but is in "high spirits", his spokesman said.

Mr Bush, 90, was admitted late on Tuesday to Houston Methodist Hospital. He received family visits on Christmas Day, when his spokesman Jim McGrath said he had "another terrific day". "President Bush remains in high spirits and continues to make progress, but he will remain at the Houston Methodist Hospital this evening," Mr McGrath said in his latest update, without explaining the reasons for the extended stay.

Late on Wednesday, McGrath had said the former president's prognosis "remains positive". The oldest of the four living former US presidents, Mr Bush, who is wheelchair-bound, seemed frail in recent public appearances.

He was treated at Houston Methodist in November 2012 for bronchitis, and ended up spending nearly two months at the hospital.

The former president's wife was admitted to the same Houston hospital nearly a year ago.

Suffering from pneumonia, the former first lady was released from the hospital in January.

Mr Bush, patriarch of one of America's most prominent political dynasties, is father to former US president George W. Bush.

Another son Jeb Bush last week announced that he was laying the foundation for his own 2016 White House bid.