MAPUTO, Mozambique—The U.S. is trying to persuade Mozambique, an African country long friendly to Moscow, to use its new seat on the United Nations Security Council to condemn Russia’s invasion of Ukraine. The American ambassador to the U.N., Linda Thomas-Greenfield, visited the southern African country this week, telling the government that neutrality wasn’t an option when a big power invades a smaller neighbor in violation of a U.N. Charter that Security Council members are bound to uphold.