The United Nations said Thursday it was assuming leadership of a U.S.-led core group of countries guiding the massive operation to provide relief to Indian Ocean tsunami survivors. Jan Egeland, the U.N. emergency humanitarian coordinator, said that the core group of the United States, Japan, India and Australia was phasing out and joining the U.N. relief operation. That group was organized by U.S. Secretary of State Colin Powell and announced by U.S. President George W. Bush on December 29, three days after an undersea earthquake off Indonesia set of a tsunami that struck 12 countries. "We will still keep a close contact with the core group because it was giving us the biggest assistance and has some of the biggest assets on the ground," Egeland said. He said that U.S. military facilities and equipment in Asia are providing crucial logistics and transport for the relief activities. The U.S. and Britain provided C-17 cargo planes, the world's largest aircraft, to carry tons of supplies to victims in many countries, particularly the Aceh and Sumatra provinces of Indonesia, the worst hit by the tsunami. --More 0005 Local Time 2105 GMT