LIMA, Dec 2 (Reuters) - Peruvian prosecutors said on Thursday they will question socialist President Pedro Castillo as part of an investigation into the promotion of certain military officers, adding to pressure on the leader as lawmakers weigh an impeachment process. Prosecutors are set to question Castillo on Dec. 14 after two ex-military heads said they were relieved of their duties after refusing to promote certain officials allegedly recommended by Castillo. The new allegations come as Castillo already faces the weakest moment of his presidency which began just four months ago, with a possible impeachment looming and a record-low approval rating of 25%, according to pollster IEP. Last week, prosecutors raided Lima"s government palace, where Castillo"s office is located, and found $20,000 in cash in a bathroom allegedly belonging to a senior adviser. Castillo fired the adviser and said his administration is committed to fighting graft. Local media also published on Sunday that Castillo had allegedly held meetings at his personal home, without disclosing them in his public agenda. Castillo denied any wrongdoing, saying the meetings had been of a personal nature. The allegations are set to add pressures to an impeachment motion presented by a group of lawmakers last week against Castillo. Congress is set to vote next week on whether to start the formal impeachment process, with only a simple majority needed to proceed. Castillo was elected earlier this year by a razor-thin margin and holds only a minority of seats in Congress. He lost against right-wing candidate Keiko Fujimori, who has said her party members in Congress will support the impeachment Castillo"s left-wing presidency has spooked markets and investors, with the local sol currency falling to record lows.