Lehman Brothers Holdings Inc. and its creditors are suing several units of Citigroup Inc. to recover $2.5 billion the failed investment bank transferred to a backup account at Citi months before seeking bankruptcy court protection.

In the complaint filed on Wednesday with the U.S. Bankruptcy Court for the Southern District of New York, Lehman claims that Citibank is wrongfully withholding the money as a potential source of funds in a dispute over derivative contracts.

Lehman also is asking the court to disallow what it says are $2 billion of "inflated and legally unsupported" claims that Citibank has asserted against it.

In a statement Thursday, Citigroup vowed to defend itself and its right to recover losses from Lehman's collapse. It called the lawsuit unjustified and accused Lehman of trying to renege on its obligations and claw back assets to which it has no right.

According to the lawsuit, Citi demanded on June 12, 2008, that Lehman transfer between $3 billion to $5 billion into an account to cover potential overdrafts by Lehman subsidiaries that were using Citi's clearing and settlement services.