(CNN) The commandant of the US Marine Corps, Gen. Robert Neller, has warned acting Secretary of Defense Patrick Shanahan that a series of "unplanned and unbudgeted" tasks -- including President Donald Trump's decisions to deploy Marines to the southern border and to reallocate a portion of the defense budget to pay for the construction of a border wall -- has imposed an "unacceptable risk to Marine Corps combat readiness and solvency."

Neller's warning came in a memo dated Monday that says the "unplanned and unbudgeted factors" have forced the Marines to cancel or reduce their involvement in several military exercises.

Though the border deployments were cited, hurricane relief efforts were by far the costliest unplanned task, at some $3.5 billion, according to Neller.

"The overall impacts of these cancellations and expected cancellations will result in three extremely negative impacts to the joint force," Neller wrote, saying those impacts would undermine efforts to maintain combat readiness, strengthen partnerships with allies and maintain solvency.

"Marines will not participate in exercises in Indonesia, Scotland, and Mongolia, and will reduce participation in exercises with Australia and the Republic of Korea at a time where we are attempting to double down on strengthening alliances and attracting new partners," he added.

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