LONDON, Nov 19 - The dollar fell to three-year lows against the yen on Wednesday and then rebounded sharply, as traders grew wary Japanese authorities may be acting to prevent the Japanese currency's rise. The dollar has been under pressure across the board on concerns about U.S. trade measures against China and the gaping U.S. current account deficit. It dropped through a previous key support level at 107.86 yen , falling towards 107.50 before turning around and rising by more than one yen in the space of a few minutes. "That was intervention I think we can safely assume," said a London-based trader. In Tokyo, Ministry of Finance officials were not immediately available for comment. The dollar was trading at the day's highs of 108.77 at 0950 GMT, off three-year lows of 107.58 hit a few minutes earlier.//