I am now a US citizen, I have an UK "life endowment policy with profits" which will shortly mature. I believe this sum is considered tax exempt in the UK. Is it also considered tax exempt in the US

Yes, it is tax-exempt, because the "dividends" from a life insurance policy is considered by the RS to merely be a refund of premiums paid and NOT true dividends.

Yes, it is tax-exempt, because the "dividends" from a life insurance policy is considered by the RS to merely be a refund of premiums paid and NOT true dividends.
Thanks for your response. Does this have to be reported to the IRS, if so what form(s) is required. Thanks again

You do NOT have to report this to anyone.