A massive, 40-mile convoy of Russian tanks and vehicles appeared to bog down on its push toward the capital of Kyiv on Tuesday while residential areas of Kharkiv, Ukraine’s second-largest city, were being pounded by Russian shells. The Russian military advance drew to within 15 miles of Kyiv’s center amid signs that troops are running out of gas and food, a senior U.S. Defense Department official said Tuesday. Russia has committed about 80% of the combat force President Vladimir Putin deployed to invade Ukraine, the official said.