The state department spokesman said the Biden administration was "deeply concerned" by the ongoing violence.
The Biden administration has joined British, Irish and Northern Irish leaders in calling for calm after Protestant and Catholic youths in Belfast hurled bricks, fireworks and gasoline bombs at police and each other.
It was the worst mayhem in a week of street violence in the region, where Britain’s exit from the European Union has unsettled an uneasy political balance.
"We are deeply concerned by the violence in Northern Ireland," said State Department Spokesman Ned Price.
"We remain, as you have heard us say before, steadfast supporters of a secure and prosperous Northern Ireland in which all communities have a voice and all communities enjoy the gains of a hard-won peace," Price told reporters.
Britain’s split from the EU has highlighted the contested status of Northern Ireland, where some people identify as British and want to stay part of the U.K., while others see themselves as Irish and seek unity with the neighbouring Republic of Ireland, an EU member.
Unrest has erupted over the past week - largely in loyalist, Protestant areas - amid rising tensions over post-Brexit trade rules and worsening relations between the parties in the Protestant-Catholic power-sharing Belfast government.