President Barack Obama flew Wednesday to Prague to join Russian President Dmitry Medvedev in signing a treaty they both have hailed as a major step forward on both arms control and U.S.-Russian relations. The formal signing of the pact, called the New Start treaty, was scheduled to take place at the medieval Prague Castle early today and is designed to bring U.S. and Russian nuclear arsenals to their smallest sizes since the early 1960s. The White House said the treaty will lower the number of long-range deployed nuclear warheads by 30 percent, although some private analysts insist the actual reduction will be much smaller. Both leaders have gotten a political boost from the treaty after years of friction between the two governments. But other subjects in the day's talks point to difficulties that still challenge the relationship. Obama and Medvedev were expected in talks to discuss ways to pressure Iran to limit its nuclear program. In the view of U.S. officials, Russia has taken a more cooperative attitude on proposed sanctions in the United Nations Security Council, but it remains unclear whether Moscow will sign on to the tough measures the Americans and other Western leaders prefer.