WASHINGTON (AFP) - US President Barack Obama on Saturday urged nations of the world to unite to resolve the most pressing global challenges - from extremism to environmental pollution to the economic crisis. In a weekly radio address timed to Easter and Passover celebrations, Obama cautioned that the world remained a dangerous place, and people must be strong and vigilant in the face of multiple threats. But let us not allow whatever differences we have with other nations to stop us from coming together around those solutions that are essential to our survival and success, he said. In a major speech in Prague, Obama heralded a far-reaching bid to abolish nuclear stocks, ban nuclear tests and halt production of fissile material. He urged the world to unite to stop terrorists killing hundreds of thousands from New York to Moscow to Islamabad to Tel Aviv. The President reiterated the same message in his radio address, saying it was evident that the current economic crisis recognized no borders, that violent extremism claimed innocent lives from New York to Mumbai and that an unsustainable dependence on foreign oil and other sources of energy was polluting the air and water. He also cited among the most pressing international concerns proliferation of dangerous weapons, deadly diseases and the recurrence of age-old conflicts. These are challenges that no single nation, no matter how powerful, can confront alone, Obama concluded. The United States must lead the way. But our best chance to solve these unprecedented problems comes from acting in concert with other nations. The President stressed that only by working together would nations be able to defeat 21st century security threats like the Al-Qaeda terror network, halt weapons proliferation and build a new foundation of mutual trust. With all that is at stake today, we cannot afford to talk past one another, Obama noted. We cant afford to allow old differences to prevent us from making progress in areas of common concern. We cant afford to let walls of mistrust stand.