Never conclusively proven, and there's a strong suspicion that the American accents were nothing more than Argentine troops with a good standard of English who'd spent some time in the US or who'd been taught the language by someone with an American accent.
As far as US cooperation at Ascension goes, there was a piece by AVM Ron Dick somwhere about his experience of how willing the Americans were to provide support - we're talking massive shipments of fuel and a refusal on their part to say 'Don't think we can' preferring to say 'Interesting challenge' shortly followed by delivery - I'll try and find it.
Edit: Page 73 onwards of
this pdf file