U.S. President Barack Obama will attend a nuclear security summit in Seoul Monday, focused on combating nuclear terrorism but likely to be overshadowed by North Korea's plans to launch a long-range rocket next month.

Leaders from more than 50 other nations and international organizations will gather in South Korea's capital for the second such nuclear meeting that was first held in Washington in 2010.

Although not on the agenda, world leaders also will likely turn their focus during the two-day summit to North Korea's recent announcement that it will launch a satellite into space using a long-range rocket. The North says this is part of celebrations marking the 100th anniversary of the April 15 birth of its late founder and President Kim Il Sung.

President Obama said Sunday he will try to urge China to use its influence over North Korea to curb such behavior when he holds a meeting on the sidelines of the summit with President Hu Jintao.

Mr. Obama said during a joint news conference with South Korean President Lee Myung-bak that China is “rewarding bad behavior” by “turning a blind eye to deliberate provocations” by North Korea.

The U.S. leader said North Korea will achieve nothing, only deepen its isolation and undermine future negotiations if it goes through with the rocket launch.

Earlier Sunday Mr. Obama visited the demilitarized zone dividing the two Koreas. The president, standing behind thick, bulletproof glass, peered across the border into North Korea with the help of powerful binoculars.

The U.N., U.S., European Union, Russia and Japan have warned North Korea that its scheduled rocket launch is in violation of U.N. resolutions, and they have urged Pyongyang to abandon the plan. Even North Korea's ally China has expressed concern that such a launch would undermine stability in the region.

The United States has said the launch would cancel an agreement to send North Korea a large shipment of U.S. food aid in exchange for halting its nuclear and long-range missile programs.

Ahead of the summit, U.S. officials announced that Ukraine has completed the removal of a consignment of highly enriched uranium, which can be used to make nuclear bombs, following a two-year program with the United States and Russia. The material, Russian in origin, has been sent back to Russia.