Heavy airstrikes by Saudi-led coalition warplanes pounded Shiite rebels and allied forces in Yemen, according to witnesses, as international agencies called for $1.6 billion to help millions of Yemenis avoid a “looming humanitarian catastrophe.” The airstrikes targeted the capital Sanaa, the southern city of Aden and the provinces of Lahj and Jouf early Friday morning, officials and witnesses said. In the northern city of Saada, the stronghold of the Shiite rebels, at least 10 civilians were killed, witnesses said. It was not immediately possible to verify the eyewitnesses account. The officials spoke on condition of anonymity because they were not authorized to speak to the press. The fighting in Yemen pits Shiite rebels, known as Houthis, and military units loyal to former President Ali Abdullah Saleh against an array of forces, including southern separatists, local and tribal militias, Sunni Islamic militants and loyalists of internationally recognized President Abed Rabbo Mansour Hadi. The rebels have controlled the capital since September.