AP Photo/Alexander Zemlianichenko, poolUNITED NATIONS (Reuters) - Russia urged "hot heads" to calm down on Friday as the United States admitted it felt "challenged" by North Korea's warning that it could test a hydrogen bomb over the Pacific and President Donald Trump and Kim Jong Un traded more insults. Trump called the North Korean leader a "madman" on Friday, a day after Kim dubbed him a "mentally deranged U.S.