Sen. Tim Kaine Timothy (Tim) Michael KaineTrump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court Barrett seen as a front-runner for Trump Supreme Court pick MORE (D-Va.) is demanding more information from the Pentagon on the attack in Niger and, more broadly, U.S. advise and assist missions around the world.

Kaine said in a letter to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE that it’s “nearly impossible” to differentiate between combat operations and advise and assist missions.

“While I fully appreciate both the necessity and importance for our Armed Forces to assist in the professionalization and capacity building of local security forces around the globe, to include those in Niger, I am concerned that our complex operating environment has made it nearly impossible to differentiate between ‘advise and assist’ and combat operations,” Kaine wrote in the letter, released Tuesday. “In turn, this makes the line triggering the requirement for congressional authorization and approval blurry.”

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The Senate Armed Services Committee, of which Kaine is a member, is scheduled to receive a classified briefing on Niger on Thursday.

Additionally, the Senate Foreign Relations Committee, on which Kaine also sits, is scheduled to hold a hearing Monday on authorizations for the use of military force (AUMF) featuring testimony from Mattis and Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE.

Earlier this month, four U.S. soldiers were killed in Niger when their team was ambushed coming back from a reconnaissance mission in the village of Tongo Tongo.

Kaine, long outspoken about the need for a new AUMF, is the latest lawmaker to question the Pentagon over the attack. Several powerful senators, including Senate Armed Services Chairman John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.), have slammed the Pentagon for not providing enough information to Congress in the wake of the ambush.

In his letter, Kaine asked whether the Pentagon plans to send more troops to Niger or another country in Africa. He also wants to know in which countries the 2001 AUMF has been used to justify military actions, what rules of engagement U.S. troops were following in Niger and how the Pentagon notifies Congress if a mission changes from advise and assist to armed conflict.

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Citing Pentagon documents, Kaine said that nearly a fifth of U.S. Special Operations Command personnel are deployed across Africa and that hundreds of operations across 20 countries in Africa are being run daily.

“Specifically, since 2013, the U.S. military personnel presence in Niger has grown from approximately 100 personnel to over 800 today making Niger host to one of the largest U.S. troop presences in Africa and raising the question of ‘mission creep,’ ” he wrote.

Those missions are being run without a specific congressional authorization, he added.

“Our system of government requires the Executive Branch to provide Congress with thorough details and information on the number of U.S. forces deployed overseas, their assigned missions and operating locations and the likelihood of further escalation of force,” he wrote, “to ensure Congress can exercise [its] constitutional obligation to authorize military action and provide critical oversight of our national security operations on behalf of the American people.”