More than 150,000 people have fled Ukraine since Russia launched an unprovoked military invasion last week, the United Nations High Commissioner for Refugees said Saturday, an exodus that the U.N. estimates could grow to as many as 5 million. The U.N.’s refugee agency said half of those fleeing had headed to the 300-mile border with Poland, where U.S. forces are helping with preparations for new arrivals, while other displaced civilians left for Hungary, Moldova and Romania. The refugee numbers are expected to intensify in the coming days as Russian ground forces push toward major cities, including the capital Kyiv with its population of 3 million people. As the sun set on the third day of the Russian assault, missiles rained down on population centers while bursts of gunfire could be heard amid intense street battles. Ukrainians crowded onto rail cars, roadways and even opted to flee on foot, sparking what many American and European officials fear is the next refugee crisis on the continent.