Airstrikes launched early on Friday by the Saudi-led Arab coalition killed at least five civilians and levelled three houses in the residential area of the old city in Yemen's capital Sanaa, Efe news agency reported.The old city has 6,500 houses that date back to the 11th century, and the area is included in the Unesco list of World Heritage Sites.Several witnesses told Efe that the three five-storey houses in al-Qasimi, the historic city's downtown, collapsed completely.Two of the residents were reported missing.Hassan al-Shamsi, one of the eyewitnesses, said that the rescue units were searching for survivors beneath the rubble.Friday's airstrike is the first time the Arab coalition has attacked houses in the old city since it launched its offensive against the Shia Houthi rebels in Yemen on March 26.