Ziraat Bank and Is Bank, Turkey's two biggest lenders, have temporarily ceased operations in Baghdad over security concerns after dozens of Turkish citizens were taken hostage by militants in the north, an executive said on Friday.Militants from the self-claimed Islamic State in Iraq and the Levant (ISIL) militants abducted 31 Turkish truck drivers as they overran the northern city of Mosul this week during a lightning advance, then seized the Turkish consulate, holding another 49 people.Iraq's most senior Shi'ite cleric on Friday urged followers to defend themselves against the advance by Sunni militants, a sharp escalation of a conflict which is threatening civil war and the potential break-up of the country.Staff working at the Baghdad branch of state-run Ziraat, Turkey's biggest bank, have been sent on leave and the bank will assess the situation again next week, the executive said on condition of anonymity.Is Bank, the country's largest listed lender, has shut its Baghdad branch until June 19, an official said.Is Bank opened the Baghdad office earlier this year. It also runs a branch in the Kurdish-run northern city of Arbil.Ziraat opened a branch in Baghdad in 2008, then a branch in Arbil, mainly serving Turkish construction firms and their workers in Iraq. The volume of loans at Ziraat's Arbil branch alone reached $215 million in February, according to a report published by the bank.