The campaign manager for presumptive Republican presidential candidate Donald Trump is to leave his job.

A campaign spokeswoman said Corey Lewandowski would no longer be working on it and said the team was grateful to him "for his hard work and dedication".

The exact reason for Mr Lewandowski's departure is not yet clear but sources close to Mr Trump have told US media he was fired.

He oversaw the New Yorker's unlikely triumph in the primary contests.

Mr Lewandowski told CNN: "I stand by the fact that Mr Trump is a great candidate and is better than Hillary Clinton will ever be."

He told the AP news agency that top Trump strategist Paul Manafort had been in operational control of the campaign since 7 April and would not divulge more about his dismissal.

But his departure comes as the businessman faces strong resistance from senior members of his own party over his strident tone, hardline immigration policy and falling poll numbers.

Among his critics is Speaker of the House and top-ranked Republican Paul Ryan, who appeared to leave the door open to a possible revolt at next month's convention.

When asked on Sunday about reports that party delegates may rebel against voting for Mr Trump at the convention, he said: "They write the rules, they make the decisions."

Mr Trump was criticised for his reaction to the Orlando nightclub shooting in which 49 people were killed.

Following the attack, he repeated his pledge that the US should ban Muslims from entering the country.