BRUSSELS—NATO defense ministers are gathering in Belgium Wednesday to begin deliberating the next phase of the Afghanistan war and to hear how military commanders plan to tamp down the insider attacks that have killed or injured 130 allied forces.

Officials here are also expected to formally announce that the top U.S. commander in Afghanistan, Marine Gen. John Allen, will be the next NATO supreme allied commander. Allen is slated to take over early next year, and Marine Gen. Joseph Dunford, the assistant Marine commandant, will take the top Afghanistan job.

During the meeting, Allen and Defense Secretary Leon Panetta plan to assure the ministers that commanders have come up with a range of ways to reduce the insider attacks in which Afghans have turned their guns on allied forces.