Spot premiums for crude and some refined products in Asia, Europe and Africa have fallen back following a U.S.-Iran deal to end the Middle East conflict, though caution about how soon normal shipping can resume is providing a floor for oil markets.Prices tumbled across the board on Monday after U.S. President Donald Trump said a preliminary agreement had been signed. Details, however, have not been made public and both the U.S. and Iran said a permanent truce is yet to be negotiated.Middle East crude benchmark Dubai's premium to swaps returned to pre-war levels of $2.06 per barrel on Monday.