WASHINGTON (Reuters) -The United States assesses that Ukraine's drone attack over the weekend hit as many as 20 Russian warplanes, destroying around 10 of them, two U.S. officials told Reuters, a figure that is about half the number estimated by Ukrainian President Volodymyr Zelenskiy.Still, the U.S. officials described the attack as highly significant, with one of them cautioning that it could drive Moscow to a far more severe negotiating position in the U.S.-brokered talks to end more than three years of war.Russian President Vladimir Putin told U.S. President Donald Trump in a telephone conversation on Wednesday that Moscow would have...