A stage cementing system includes a stage cementing assembly having a stage tool. The stage tool has an outer mandrel, an inner mandrel coupled to and disposed inside of the outer mandrel, an annular chamber between the outer mandrel and the inner mandrel, a first outer port through the outer mandrel, and longitudinally spaced first and second inner ports through the inner mandrel. The stage cementing system further includes an inner string assembly configured to be located inside the inner mandrel. The inner string assembly has a tubular body having a central throughbore and longitudinally spaced first and second side ports, a lower external seal element below the first and second side ports, a middle external seal element between the first and second side ports, and an upper external seal element above the first and second side ports.