Presumptive Republican presidential candidate Donald Trump unveiled an "America first" energy plan he said would unleash unfettered production of oil, coal, natural gas and other energy sources to push the United States toward energy independence.

Mr Trump promised on Thursday to cancel the Paris climate agreement and stop all payments of US tax money to a United Nations fund to mitigate effects of climate change worldwide.

But the speech, delivered at the annual Williston Basin Petroleum Conference in Bismarck, North Dakota, went far beyond energy, as Mr Trump laid out, in his most detail to date, a populist general election pitch against likely rival Hillary Clinton.

"She's declared war on the American worker," Mr Trump said of Mrs Clinton, reading from prepared remarks in a stadium packed with thousands.