BELFAST, Northern Ireland  — Protestant youths hurled Molotov cocktails at police in Northern Ireland, wounding at least 27 officers in a fourth night of protests over restrictions on traditional marches, as the White House expressed "deep concern" about the violence.

The regional parliament, recalled from its summer recess, was due to meet on Tuesday to discuss ways of restoring order.

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Police said they responded with water cannons and at least one baton round in Belfast. Protests and other incidents were reported in at least five other towns across the British province.

Thousands of pro-British Protestants march every summer, a regular flash point for sectarian violence as Catholics, many of whom favor unification with Ireland, see the parades as a provocation.

Protestant marchers, unhappy because authorities ruled they could not walk along a stretch of road that divides the two communities, started throwing bricks and bottles on Friday.

Vice President Joe Biden expressed his "deep concern" about the violence during a phone call with Northern Ireland's leaders on Monday, the White House said.

The police service called on the regional parliament to help end the clashes.

"Today is a day for calming words and a renewed commitment from the Assembly to finding political solutions," the force's chief constable Matt Baggott said.

"There are already too many injured police officers and young people facing prison sentences for anything else to be acceptable."

At least six home-made explosive devices and several petrol bombs were thrown at police in east Belfast on Monday night, police said.

Catholic protesters clashed with police in one protest in the Adroyne area of the city, the force added. One car was set alight during clashes in north Belfast.

Protests and other incidents were also reported in the towns of Newtownabbey, Antrim, Dungannon, Portadown and Londonderry.

A 1998 peace deal mostly ended decades of sectarian strife in the British province but trouble still breaks out, particularly around the Orange parades which mark a 1690 Protestant victory over a Catholic king.

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Police said they had brought hundreds of reinforcements from Britain. Sixty people have been arrested and 71 officers wounded since the latest clashes started.