Afghan forces have been moving out of Kabul to put down a flare-up of rebellion in the provinces, leaving the capital's security to Soviet troops, Western diplomatic sources said. The sources had no estimate of the number of Afghan troops involved. But they quoted sources in Kabul as saying all four central corps divisions were being sent out. Fighting has broken out in at least 20 of Afghanistan's 29 provinces, many of them bordering the Soviet frontier, the sources said. They said fresh Soviet troops were reported to have arri ved recently in Kabul.