GMAC, the finance unit once controlled by General Motors Corp, said on Tuesday Citigroup will double its asset-backed funding facilities to $21.4 billion as it struggles with subprime mortgage losses.

The facilities replace an existing $10 billion facility from August 2006, and will include commitments to provide funding for U.S. automobile-related assets, mortgage assets, and other assets across GMAC and its subsidiaries.

The financing company has suffered subprime mortgage losses at its home lending unit, ResCap. The losses forced GMAC to post a 63 percent decline in second-quarter profit.

Like many lenders, ResCap has struggled as falling home prices and rising interest payments have made it tougher for many homeowners to keep up with their mortgage payments.

A total of $14.4 billion will become available for immediate funding, with the additional $7 billion becoming available if certain conditions are met, GMAC said in a filing with the Securities and Exchange Commission.

In November, GM sold a majority stake in GMAC to a group led by Cerberus Capital Management. It still has a 49 percent stake in GMAC. (Reporting by Jui Chakravorty)