New York (CNN Business) The State Department has yet to explain or defend its highly unusual decision to remove an NPR reporter from an upcoming trip by Secretary of State Mike Pompeo at a time when Pompeo is fuming about the public radio network.

NPR's diplomatic correspondent Michele Kelemen was supposed to be the radio representative on the plane that will travel with Pompeo to the United Kingdom, Ukraine, Belarus, Kazakhstan, and Uzbekistan in the coming days. He departs for the UK on Wednesday.

On Tuesday afternoon NPR CEO John Lansing said the State Department "has been unresponsive" to its inquiries.

In a new letter, Lansing and senior Vice President of News Nancy Barnes asked Pompeo to reverse the department's decision and allow Kelemen on the trip; to explain the original decision; and to provide all relevant documents.

Separately, in a memo to NPR staffers, Lansing pointed out that "there are serious issues at stake."

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