Masked gunmen opened fire on an army bus in Cairo on Thursday, killing one officer and wounding three soldiers, a military source told Reuters.
Islamist militants are expanding their insurgency in Egypt where army chief Field Marshal Abdel Fattah al-Sisi, who overthrew Mohamed Mursi of the Muslim Brotherhood in July, is expected to announce he will run for president within days.
Militants based in the Sinai peninsula near the Israeli border have stepped up attacks on soldiers and policemen since Mursi's ouster, killing hundreds and spreading their campaign to Cairo and other cities.
A security crackdown has devastated the Brotherhood, driving Egypt's most organized political group underground.
But Sinai-based militants are still a major security threat to the Arab world's biggest nation despite army offensives which include air strikes.