US President Donald Trump has blamed wet weather and a teleprompter fault for his reference to 18th century airports in his Independence Day speech.

Mr Trump told rain-soaked crowds that American troops fighting the British "took over the airports" during the American Revolutionary War in the 18th century.

Observers were quick to point out that airports did not exist at the time, as the aeroplane was yet to be invented.

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But Mr Trump said the slip-up was due to a technical failure.

"The teleprompter went out and kept going on and then at the end it just went out. It went kaput," Mr Trump said.

"So I could have said that and actually right in the middle of that sentence it went out. And that's not a good feeling when you're standing in front of millions of millions of people on television.

"I knew the speech very well, so I was able to do without a teleprompter but the teleprompter did go out and it was actually hard to look at anyway because it was rain all over it.

"But despite the rain that was just a fantastic evening."

Trump supporters as well as military families were given VIP tickets to the parade. ( Reuters: Carlos Barria )

The Revolutionary War pitted British forces against American colonists who wanted independence from the Crown.

But it ended in 1783, some 120 years before the American Wright brothers invented the aeroplane — a discrepancy quickly pointed out on Twitter.

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Elsewhere in his speech Mr Trump strove to strike a unifying tone, declaring that the "future of American freedom" lay with those who would defend it.

Mr Trump said "there will be nothing that America cannot do", asking his compatriots to remember the nation's history and "never stop fighting for a better future."