Ukrainian forces have made “notable progress” in their counteroffensive over the last 72 hours, notching wins against the second line of Russian defenders, a White House spokesman, John F. Kirby, said at a news conference on Friday. The United States, Mr. Kirby said, will not discuss Ukraine’s war plans, or how its forces would exploit their gains. But, he added, the U.S. expects Ukraine to continue to push further south, requiring “tough fighting ahead.” Ukraine’s military celebrated a tactical victory earlier this week when its forces retook the southern village of Robotyne. Days later, Ukrainian assault units moved east toward...