A top U.S. bank regulator warned weakened institutions not to engage in aggressive growth strategies that rely on volatile liabilities or the government’s expanded deposit insurance and debt guarantees.

The Federal Deposit Insurance Corp, in a letter to the banks it regulates, said on Tuesday that such strategies pose a significant risk to the agency’s deposit insurance fund, and that banks engaging in them will be subjected to heightened supervisory review and enforcement.

The FDIC is trying to protect its insurance fund, which took a big hit during the fourth quarter of 2008, plunging almost 50 percent to $18.9 billion as it set aside billions of dollars for actual and expected bank failures.

Last week the FDIC approved a package of measures aimed at raising as much as $27 billion this year in assessment revenues that will replenish the fund, including $15 billion from a one-time fee in the third quarter.

The agency said in its letter that banks that engage in aggressive growth strategies or rely excessively on a volatile funding mix will be subjected to more stringent supervision and may face higher deposit insurance premiums.

The FDIC said prudent lending practices generally would not be considered as increasing the risk profile.

The agency has been trying to strike a careful balance, making sure that banks hold significant capital cushions while maintaining healthy lending levels.