This is an undated hand out photo of Charlie Gard provided by his family, at Great Ormond Street Hospital, in London. Picture: Family of Charlie Gard via AP

London - US President Donald Trump on Monday offered to help a sick British 10-month-old after the infant's parents were legally blocked from bringing him to a treatment trial in the United States.

Charlie Gard, widely known as Baby Charlie, suffers from a rare condition that causes progressive muscle weakness and brain damage.

"If we can help little Charlie Gard, as per our friends in the UK and the Pope, we would be delighted to do so," Trump said on Twitter, referring to a message of support from Pope Francis on Sunday.

Specialists at London's Great Ormond Street Hospital had told parents Chris Gard and Connie Yates the therapy proposed by a US doctor for their son is experimental and will not help, and that life support for the child should be stopped.

British and European courts had upheld lower court judgements ruling that the infant's life support should be ended so that he could die with dignity.

On Friday, Great Ormond Street Hospital granted the infant extra time on life support but was expected to turn off life-sustaining treatment in the coming days.

It was unclear from Trump's tweet what kind of help he was offering the family.

"Thanks @realdonaldtrump for your support," Gard and Yates wrote on their official Charlie's Fight Twitter account, urging British Prime Minister Theresa May to "do the right thing and save Charlie Gard."