By David Milliken and Jemima Kelly LONDON (Reuters) - The United States urged international creditors to show more flexibility in negotiations with Greece's cash-strapped government to avert a possible Greek default and exit from the euro zone with incalculable consequences. U.S. Treasury Secretary Jack Lew issued the warning on Wednesday in a stopover in London on his way to a meeting of Group of Seven finance ministers in Dresden, Germany. "My concern is not the goodwill of the parties -- I don't think anyone wants this to blow up -- but ... a miscalculation could lead to a crisis that would be potentially very damaging," Lew told students at the London School of Economics.