US President Donald Trump has offered to help a terminally ill British baby, saying on Twitter: "If we can help little #CharlieGard, as per our friends in the UK and the Pope, we would be delighted to do so."

The baby, 10-month-old Charlie Gard, has been at the centre of a long legal battle between his parents, who wanted him to undergo an American therapy trial, and specialists at the hospital in London, who said the treatment was experimental and would not help.

Charlie suffers from a rare genetic condition that has left him unable to move his arms, legs or to breathe unaided.

He has a form of mitochondrial disease — a genetic condition that causes progressive muscle weakness and brain damage.

The baby's parents, Chris Gard and Connie Yates, launched a fundraising appeal to help pay for his doctors' bills in the United States.

They have raised 1.3 million pounds ($AU2.2 million) from more than 83,000 donations, according to his Gofundme page.

Charlie suffers from a rare genetic condition that has left him unable to breathe unaided. ( GoFundMe: Charlies Fight )

Britain's Supreme Court ruled last month that going to the United States for treatment would prolong the baby's suffering without any realistic prospect of helping him.

The court would not allow an American doctor found by the couple to be identified, and details about the therapy were not available.

The parents asked the European Court of Human Rights to overturn the ruling, but that court last week declined to intervene.

His parents and a London children's hospital said on Friday the boy would be given "more time" before life support was withdrawn.

Court ruling means little Trump can do

White House spokeswoman Helen Aguirre Ferre said members of the Trump administration had spoken to the Gard family.

"Although the President himself has not spoken to the family — he does not want to pressure them in any way — members of the administration have spoken to the family in calls facilitated by the British Government," she said, calling the situation "heartbreaking".

"The President is just trying to be helpful if at all possible."

It was unclear how Mr Trump would propose to help, and there is little he can do, because UK and European courts have deferred to the hospital's decision not to allow Charlie to be sent to the US for trial therapy.

Mr Trump waded in to the case as his fellow Republicans in the US Senate struggled to reach consensus on a healthcare reform plan that would slash health spending for low-income Americans.

Pope Francis on Sunday called for Gard's parents to be allowed to do everything possible to treat their child.

In a statement, the Vatican press office said Pope Francis, "is following with affection and sadness the case of little Charlie Gard and expresses his closeness to his parents".

"For this he prays that their wish to accompany and treat their child until the end is not neglected," the statement read.

UK Prime Minister Theresa May's spokesman, asked about Mr Trump's tweet, said it was a very sensitive case and their thoughts were with Charlie's family.

AP/Reuters