* Saudi kings gives $1 bln to Lebanese army * Mayor of Arsal says civilians are suffering * 17 soldiers killed during incursion BEIRUT, Aug 6 (Reuters) - The Lebanese army and Islamist militants clashed on Wednesday despite a 24-hour ceasefire agreed to end five days of fighting that has killed dozens of people in the most serious spillover of Syria's three-year civil war into Leba**n. Saudi Arabian King Abdullah granted $1 billion to help the Lebanese army to bolster security as they battle militants who have seized the border town of Arsal on the Syrian frontier, state news agency SPA reported.