The United Nations has ordered all of its non-essential international staff to leave Syria, saying Monday that the escalating violence in the civil war-struck country is making it harder and more risky for humanitarian workers to do their jobs. The U.N. also plans to reduce some of its field work in the Arab state.

More than 40,000 people have been killed since Syrians began a revolt against President Bashar Assad in March of last year. Dozens of aid workers have been caught in the crossfire and killed.

Officials said up to a quarter of the 100 international staff working for several U.N. agencies could exit by week’s end.