By Hani Amera ON BOARD THE MORNING GLORY, Libya (Reuters) - A commercial oil tanker seized by U.S. forces after it loaded crude at a Libya port held by anti-government rebels has docked back in the capital Tripoli, a Reuters witness said on Sunday. U.S. special forces boarded the tanker a week ago off Cyprus, days after it left Es Sider port, which is controlled by rebels who demand more autonomy and oil wealth in defiance of the central government. A Reuters reporter was allowed to board the Morning Glory moored near the Tripoli coast, witnessing how Libyan navy forces arrested the 21-strong crew and three rebels who had boarded the ship at Es Sider. It was a rare victory for Tripoli, which is struggling to end a port blockade by rebels, one of many challenges facing the central government which has failed to secure the North African country three years after the fall of Muammar Gaddafi.