Logistics problems are stalling a massive Russian convoy that's pushing its way toward Kyiv, according to a senior U.S. defense official. The convoy, which has been measured as stretching for 40 miles, is apparently being hampered by fuel and food shortages. The news comes as Russia continues to concentrate attacks on the large Ukrainian cities of Kyiv and Kharkiv.As night fell on Ukraine on Tuesday, Russia's large convoy was still about 18 miles north of Kyiv — representing little or no change from Monday, the official said. The official added that some elements within the military column are "literally out of gas" and having difficulty feeding their troops."The U.S. says about 80% of the estimated 190,000 Russian troops that rimmed Ukraine are now in the country," NPR Pentagon correspondent Tom Bowman reports. "About 400 missiles have been fired by the Russians since the invasion began last week."The official told reporters it's possible that the Russians are now regrouping and their leaders are rethinking their invasion strategy. Since the war began last week, Ukraine's military — and its regular citizens — have repeatedly shown a willingness to engage with the Russian force. Despite Russia's vastly larger armed forces, Ukraine's airspace remains contested, with President Volodymyr Zelenskyy's country currently mounting a viable defense, the senior defense official said.As in the first days of the war, Russia's military appears to be having more success in southern Ukraine, the official said. Copyright 2022 NPR. To see more, visit https://www.npr.org.