Washington (CNN) Sen. Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, wants answers from Secretary of State Mike Pompeo on the presence of US troops in Niger after four American service members were killed during an ambush by ISIS fighters there in October.

CNN has obtained a copy of the letter Menendez sent Tuesday to Pompeo inquiring about US policy and objectives in Niger as President Donald Trump's top diplomat prepares to brief the entire committee on the issue this week.

"The State Department has responsibility for formulating and executing US foreign policy. Congress has a responsibility to ensure that military operations that put our men and women in uniform in harm's way support our diplomatic effort," the letter said.

"I hope you will be able to provide the Committee with a full understanding of our efforts in Niger, including the authorities under which US personnel are are operating," it said.

Specifically, Menendez wants Pompeo to answer the following questions:

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