Freddie Mac ( FRE) said Monday that it will request an additional $30 billion to $35 billion in government aid, and that CEO David Moffett has resigned from the struggling mortgage finance giant six months after taking over the job.

Moffett took over leadership of Freddie Mac in September, after it and its counterpart Fannie Mae ( FNM) were placed into conservatorship by the federal government due to accelerating losses on bad mortgage debt. Freddie's board of directors is searching for a replacement with the Federal Housing Finance Agency, which oversees the company.

Moffett, former CFO of US Bancorp ( USB) will leave Freddie Mac by March 13. Chairman John Koskinen said Freddie plans to name an interim replacement before he departs.

Koskinen said board members "are very sorry to see David go," adding that he made "valuable contributions" as it was placed into conservatorship. FHFA Director James Lockhart also thanked Moffett "for his service and leadership" through the transition period and said FHFA would work with the company to ensure a smooth transition to a new CEO.

Freddie said that Moffett would like to return to the financial services sector, but did not provide specific details of his future plans.

Freddie also said it will ask for another $30 billion to $35 billion from the Treasury Department in exchange for senior preferred stock after it files its annual report. The government agreed to provide Fannie and Freddie with billions of dollars worth of capital over time, in exchange for stock and ownership rights, to keep the firms solvent and prevent further chaos in the mortgage market.

Fannie Mae said last week that it lost $25.2 billion in the fourth quarter -- bringing its annual loss to $58.7 billion -- and will ask for another $15.2 billion in government aid. The two firms hold or guarantee more than half of the U.S. mortgage market.