Republican presidential hopeful Donald Trump has asked Indiana Governor Mike Pence to be his running mate, and Pence has agreed, US networks reported on Thursday.

CNN and ABC both reported, without naming their sources, that Pence, a 57-year-old Christian evangelical and stalwart of the American right, had accepted the vice presidential slot.

Trump will formally announce a decision on his choice of running mate on Friday, with three days to go until Republicans gather in Cleveland, Ohio to officially select their White House nominee.