The United States is crafting a plan for a drawdown of staff from the US embassy in Havana in response to still-unexplained incidents that have harmed the health of some US diplomats there, US and congressional officials said.

The State Department's plan, which was being finalised and could be announced within days, would call for the departure of non-essential staff and diplomats' families, the sources said.

But it was unclear whether the withdrawal would be voluntary or compulsory, and one US official said some details were still being worked out.