Donald Trump compared immigrants to a treacherous snake that bites its host as he commemorated his 100th day as President.

Mr Trump read the lyrics of Al Wilson's 1968 song "The Snake," in which a woman takes an ill snake into her home only for it to bite her once it has recovered.

He also paused to point out to his audience it was a metaphor for "the border."

During his speech to a rally in Harrisburg, Pennsylvania, Mr Trump dedicated his recital to Secretary of Homeland Security John Kelly and anti-immigration agencies.

He said: "So let's dedicate this to General Kelly, the Border Patrol, and the ICE agents for doing such an incredible job. All right?

US President Donald Trump attacks media at 100-day rally in Pennsylvania

"This was written by Al Wilson, a long time ago. And I thought of it having to do with our borders and people coming in and we know what we're going to have.

"We're going to have problems. We have to very, very carefully vet. We have to be smart. We have to be vigilant. So here it is, 'The Snake.' It's called 'The Snake.'

"On her way to work one morning, down the path along the lake, a tender hearted woman saw a poor, half frozen snake. His pretty coloured skin had been all frosted with the dew. 'Poor thing,' she cried, 'I'll take you in, and I'll take care of you.'

Here, Mr Trump paused to explain: "The border."

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He continued: "Take me in oh, tender woman. Take me in, for heaven's sake. Take me in, oh, tender woman, sighed the vicious snake.

"She wrapped him up all cozy in a comforter of silk, and laid him by her fire side with some honey and some milk.

"She hurried home from work that night and as soon as she arrived, she found that pretty snake she'd taken in had been revived.

"Take me in, oh, tender woman. Take me in for heaven's sake. Take me in oh, tender woman, sighed that vicious snake.

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"She clutched him to her bosom, 'you're so beautiful', she cried.

"'But if I hadn't brought you in by now, oh, heavens, you would have died.'

"She stroked his pretty skin again and kissed him and held him tight. But instead of saying thank you, that snake gave her a vicious bite!

"Take me in oh, tender woman, take me in for heaven's sake. Take me in, oh, tender woman, sighed the vicious snake.

US President Donald Trump addresses a 'Make America Great Again' rally in Harrisburg ( AFP )

"'I have saved you,' cried the woman. 'And you've bitten me, heaven's why? You know your bite is poisonous, and now I'm going to die.'

"'Oh, shut up, silly woman,' said the reptile with a grin. 'You knew damn well I was a snake before you took me in.'"