The MoD has sent written guidance to coroners across England and Wales over the holding of military inquests.
According to the Times, its letter says the US "confirms categorically" it will not provide witnesses for inquests.
It comes six days after three British soldiers were killed by US "friendly fire" in southern Afghanistan.
The Times reports that the letter to coroners states: "The US have confirmed categorically that they will not provide witnesses to attend UK inquests.
"While coroners may continue to ask for US witnesses to attend... they should be aware that there will in all cases be a refusal."
The three soldiers who died on Thursday were Privates Aaron McClure and Robert Foster, both 19, and John Thrumble, 21, of the 1st Battalion The Royal Anglian Regiment.
They had been on patrol in Helmand province when US fighter planes dropped a 500lb bomb, killing them and injuring two other soldiers. The US State Department is investigating the deaths.
In March, Oxfordshire assistant deputy coroner Andrew Walker was critical of the failure of the US authorities to co-operate at the inquest of Lance Corporal of Horse Matty Hull.
L/Cpl Hull, 25, died when a US pilot fired on his convoy in Iraq in 2003. No American witnesses gave evidence at the inquest.
The coroner said at the time: "I find the decision of the US authorities not to allow the relevant persons to attend to give evidence, or to themselves provide full transcripts of questions those people were asked during the Friendly Fire Investigation Board, hard to understand."
Nah, not at all hard to understand, Lord Coroner, not at all. The men in question were killed by military ordnance. Regrettably. For one, you wouldn't understand one blithering bit of what they were talking about, and for another this is the type of information that is quite sensitive and needn't fall into the wrong hands, as operational stratagems and performance capabilities of various weapons systems are not something one parades about whenever an amateur thinks he's a need to know.
You do not. We don't send our military people to international inquests, but suffice to say our people are working feverishly with your people to see that this does not happen very often. Can't say it won't happen ever again, because in war these horrible circumstances rear their ugly heads at the behest of Mr. Murphy, and he sir, is not someone that ANYONE can reason with.