US President Donald Trump on Saturday again opposed the idea of a united European military force and asked NATO allies to pay their "fair" share for their defence.

"The idea of a European military didn't work out too well in WW I or 2. But the US was there for you, and always will be," Trump said in a tweet.

His comments came amid moves by certain European countries to have a combined European military so as to reduce their dependence on US forces in the region.

"All we ask is that you pay your fair share of NATO. Germany is paying 1 per cent while the US pays 4.3 per cent of a much larger GDP - to protect Europe. Fairness!" Trump said.

Ever since coming to power, Trump has been demanding that European NATO allies increase their contribution to NATO to two per cent of their GDP.

However, several of his followers on Twitter were quick to point out that there was no joint European military during World War I or II.

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