Police fire water cannons at Belfast protesters

The U.S. has expressed concern about renewed violence in Northern Ireland after another night of unrest.

A crowd of young people Thursday evening threw petrol bombs and stones at Belfast police who responded with water cannons.

A week of violence has injured 55 police officers and seen boys as young as 13 arrested on rioting charges.

U.S. State Department spokesman Ned Price, "As the United Kingdom and the EU implement Brexit-related provisions, this administration encourages them to prioritize political and economic stability in Northern Ireland."

The violence is fueled in part by frustration among pro-British unionists over post-Brexit trade barriers.

Unionists say the so-called Northern Ireland protocol amounts to a border in the Irish Sea, and they feel cut-off and betrayed by London.

There's also anger about the failure to prosecute members of the Irish nationalist party Sinn Fein, who allegedly breached lockdown rules, when they attended the funeral of a former IRA member in June last year.

In turn, Sinn Fein has accused the pro-British Democratic Unionist Party, or DUP, of stoking tensions by opposing new trading barriers.

Both parties put their differences aside on Thursday to condemn the violence.

Washington joined in support of the 1998 Good Friday Agreement, which ended decades of violence.

"President Biden has been unequivocal in his support for the Belfast and Good Friday agreement, which was an historic achievement. We believe that we must protect it and we believe that we must ensure it doesn't become a casualty of Brexit."

Much of Northern Ireland remains deeply split along sectarian lines, 23 years after the Good Friday deal.

Many Catholic nationalists aspire to unify with Ireland, while Protestant unionists want to stay in the UK.