(Reuters) - United Continental will furlough about 685 flight attendants, as the U.S. airline company looks to prune its expenses as part of a $2 billion annual cost-cutting plan announced last year. United said it failed to attract enough takers for its voluntary furloughs and a job share-partnership program, leading it to undertake involuntary furloughs. The airline will furlough some of the Association of Flight Attendants' (AFA) "most junior members," the union, which represents over 25,000 United flight attendants, said in a statement. "What it comes down to is simply the promise of United's merger not being realized," said Greg Davidowitch, AFA President at United. Chicago-based United has struggled to consolidate its operations nearly three years after the merger of UAL Corp and Continental Airlines for $3.17 billion. It has been working to win back customers after it faced severe service-related and operational problems last year. United Continental said in November it would cut costs by $2 billion every year through measures including increasing productivity, reducing sourcing costs, improving maintenance processes and inventory procedures, and optimizing distribution methods. "Successful airlines do not lay off workers," Davidowitch said. "We continue to meet with management and offer creative solutions to an involuntary furlough; while also addressing the company's needs to mitigate an overage in manpower." (Reporting by Sruthi Ramakrishnan in Bangalore; Editing by Supriya Kurane)