"One positive sign is that the administration wants to invest more resources on reducing this backlog," he said. But Washington needed to find ways to manage the flow "without penalising the people themselves, people who oftentimes have very valid reasons to seek asylum".
"On measures that result in separating children from their families, we are very concerned and have expressed this concern," he said.
Grandi said governments in many rich countries had adopted "despicable" rhetoric on migration, ignoring their duty to help people fleeing war or persecution. UNHCR's annual report, published on Tuesday, showed the global number of refugees grew by a record 2.9 million in 2017 to 25.4 million, with another 43.1 million people forcibly displaced in their own countries.
The vast majority of refugees remain in poor countries, with only small numbers seeking refuge in the West.
"We're not talking about unmanageble numbers moving to the rich countries," he said.
Governments should explain why it is right to help refugees, "but the contrary has happened. Governments have projected an image of emergency, of invasion, and actually, unfortunately many political leaders have capitalised on that to gain votes."