Former US President George HW Bush, who has been hospitalised for more than a month, "continues to improve" but remained in intensive care at a Texas hospital, a family spokesman said.
"The president is alert and, as always, in good spirits -- and his exchanges with doctors and nurses now include singing," family spokesman Jim McGrath said.
Physicians remained "cautiously optimistic that the current course of treatment will be effective", McGrath added.
Bush (88), the oldest living former US president, has been hospitalised since November 23. He was moved into intensive care on Sunday for treatment of a fever following a bronchitis-related cough.
Longtime aide Jean Becker said in a statement on Thursday evening that Bush likely would advise well-wishers to "put the harps back in the closet".
Becker said "most of the civilised world" contacted her on Wednesday after disclosures that Bush had been placed in the intensive care unit after physicians were having difficulty bringing a fever under control.
"Someday President George HW Bush might realise how beloved he is, but of course one of the reasons why he is so beloved is because he has no idea," Becker said in the at-times lighthearted statement that made multiple references to the former president's sense of humour.
She said updates about Bush's condition have been limited "out of respect for President Bush and the Bush family who, like most of us, prefer to deal with health issues in privacy". She said another factor was "because he is so beloved we knew everyone would overreact".
"I hope you all know how much your love, concern and support are appreciated," Becker said.
While the president's treatment was "unequalled anywhere", she said prayers also were needed and welcomed.
It had been hoped Bush would be well enough to spend Christmas this week at home. But while his cough improved, he developed a persistent fever.