The US will take the lead in ridding the world of nuclear weapons, President Barack Obama has promised. Speaking in front of a crowd of thousands in Prague, Mr Obama said the US was ready to take "concrete steps" to end what he called "the most dangerous legacy of the Cold War". The US will negotiate a new arms reduction treaty with Russia this year, he announced, saying he had already begun to lay the groundwork for a new deal with Russian President Dmitri Medvedev. Mr Obama said the United States would maintain a safe and effective arsenal to deter any adversary, but will begin reducing its arsenal. He said North Korea broke the rules with its rocket launch earlier on Sunday and must be forced to change. Referring to the threat of terrorism, Mr Obama announced a new international effort to "secure all vulnerable nuclear material around the world" within four years. But he said the United States will continue pushing plans for a missile defense shield stretching across Europe as long as a potential nuclear threat persists from Iran. Mr Obama said: "If the Iranian threat is eliminated, we will have a stronger basis for security, and the driving force for missile contruction in Europe will be removed." The Czech Republic is one of two sites in central Europe earmarked for the planned missile shield, which has upset Russia.