US President Donald Trump offers lifeline to terminally ill British baby Charlie Gard.

US President Donald Trump has offered help to a terminally ill infant an ocean away in a Twitter post after his parents lost a legal battle to give him experimental therapy in the United States.

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.

“If we can help little #CharlieGard, as per our friends in the UK and the Pope, we would be delighted to do so,” Trump said in a tweet posted on Monday.

The baby, who turns 11 months old on Tuesday, suffers from a rare genetic disorder 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.

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

It was unclear how the US president would propose to help in the case.

The baby's parents, Chris Gard and Connie Yates, launched a fundraising appeal to help pay for his doctors' bills in the United States. To date, they have AU $2.2 million from more than 83,000 donations, according to his GoFundMe page.

Last month, Britain's Supreme Court ruled 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.

Find out more about Charlie's illness via the GoFundMe page launched by his parents here.