Citigroup, the US bank, has acted to shore up six of its hedge funds that have been hit by worsening conditions in the municipal bond market. The bank has committed to injecting $1 billion across six highly leveraged municipal bond funds with $15 billion in assets, according to the New York Times. About $600 million had been provided as of last week, according to people briefed on the situation, after lenders issued a margin call following a sharp fall in securities values. The move is the latest step by the chief executive, Vikram Pandit, to stablise the fortunes of the bank, which has been hit hard by the credit crisis.