John Phelan, the civilian head of the U.S. Navy, is leaving his position effective immediately, the Pentagon announced Wednesday, marking a sudden leadership transition within the military branch.

Pentagon spokesman Sean Parnell confirmed the departure in a public statement, saying Phelan is “departing the administration” without providing additional details about the circumstances surrounding the decision.

Hung Cao, who has been serving as Navy undersecretary, will assume the role of acting secretary, taking over day-to-day leadership responsibilities at the Department of the Navy.

The announcement came just one day after Phelan appeared at the Navy’s annual conference in Washington, where he addressed sailors and defense industry leaders and spoke with reporters about his policy agenda and priorities for the service.

As Navy secretary, Phelan served as the top civilian overseeing the branch’s operations, budget, and strategic direction, working in coordination with Pentagon leadership and Congress.

No reason for his departure was immediately provided, and officials have not indicated whether a permanent replacement has been selected. The acting appointment allows leadership continuity while the administration considers its next nominee for the position.