(Adds more comments, background) WASHINGTON, March 4 (Reuters) - Iran's foreign minister said in a U.S. television interview on Wednesday he believed "we are very close" to a nuclear arms deal with Western powers, but cautioned there were details that needed to be worked out. "We are prepared to work round the clock in order to reach an agreement," Mohammad Javad Zarif told NBC News in an interview excerpt released by the network.