BRUSSELS — NATO defense ministers are considering a new proposal to sustain Afghanistan’s security forces at 352,000 troops through 2018, senior alliance officials said Thursday. The expensive effort is viewed as a way to help guarantee the country’s stability — and, just as much, to illustrate continued foreign support after the NATO allies end their combat mission in Afghanistan next year.

The fiscal package that NATO leaders endorsed last spring would have reduced the Afghan National Security Forces to fewer than 240,000 troops after December 2014, when the NATO mission expires. That reduction was based on planning work indicating that the larger current force level was too expensive for Afghanistan and the allies to keep up, and might not be required. Some specialists even said that the foreign money pouring into Afghanistan to support so large a force was helping fuel rampant official corruption.