By Phil Stewart WASHINGTON (Reuters) – Russia’s combat power in Ukraine has declined below 90 percent of its pre-invasion levels for the first time since its attack began, a senior U.S. defense official said on Tuesday, suggesting heavy losses of weaponry and growing casualties. The United States has estimated Russia assembled more than 150,000 troops around Ukraine before the Feb. 24 invasion, along with enough aircraft, artillery, tanks and other firepower for its full-scale attack. “For the first time they may be just a little bit below 90 percent,” the U.S. defense official told reporters …