TELFS-BUCHEN, Austria—The Group of Seven leaders are expected to agree to start work on a mechanism to cap the purchase price of Russian oil, with the U.S. also announcing a new round of military aid for Ukraine and sanctions against Moscow, Biden administration officials said Monday. Leaders will direct relevant ministers in their countries to work on the details of Russian oil caps, which would create a buyers’ cartel of Western nations and their allies, the official said. There is no timeline yet on when the details will be worked out. The issue was discussed by G-7 leaders on Sunday,...