The conference had been scheduled to take place in Madison, Wisconsin.
Samba Baldeh, a native of Gambia and an elected official in Madison, says:
This is part of a broad policy of the Trump administration to deny, stall and obstruct visa requests regardless of their source.”
African ambassadors to the US from Botswana, Ghana and South Africa were listed as speakers at the event, as was the co-ordinator of former President Obama’s Power Africa programme, the news site says.
By denying them entry, the US has “squandered a great opportunity for energy-deprived nations to work with advanced energy providers of the US and Wisconsin,” Mr Baldeh added in comments reported by the Wisconsin State Journal.
One of the organisers, Mary Flowers, told Voice of America that the past three events went ahead despite around 40% of would-be participants being denied visas.
But she added:
This year it was 100 per cent. Every delegation. And it was sad to see because these people were so disheartened.”