Citigroup on Friday moved to bolster its strained balance sheet by selling $12bn of government-guaranteed bonds, the largest issuance since the US authorities agreed to backstop financial companies’ short-term debt in November. The move – the second time Citi has tapped the government-backed facility – comes as the group tries to get back on a firmer footing after recording more than $50bn in credit-related losses since the start of the crisis.