By Michelle Nichols UNITED NATIONS (Reuters) - Cuba's President Raul Castro, speaking at the United Nations for the first time, on Saturday hailed renewed diplomatic ties with the United States as "major progress" but slammed a U.S. trade embargo as the top obstacle to Cuba's economic development. Castro and U.S. President Barack Obama in December announced a detente following more than half a century of animosity between the former Cold War foes that culminated in the restoration of diplomatic relations on July 20.