Rioters have been blasted with water cannon in fresh unrest on the streets of west Belfast.
It comes after Northern Ireland Secretary Secretary Brandon Lewis admitted Brexit had created "real issues" in the country, but said violence was not the answer.
Mr Lewis travelled to the city following riots across Northern Ireland that have involved children as young as 12 and left 55 police officers injured.
Joe Biden’s White House has called for calm.
White House press secretary Jen Psaki said the Biden administration was concerned by the violence, "and we join the British, Irish and Northern Irish leaders in their calls for calm".
Police have called it the worst unrest in years.
On Thursday night, trouble flared again when stones and fireworks were thrown at police by youths on the nationalist Springfield Road, close to the scene of Wednesday’s riots.
Water cannon was deployed after warnings to disperse were ignored and people continued to throw missiles.
Irish Prime Minister Micheal Martin and Prime Minister Boris Johnson called for calm after they spoke over the phone on Thursday about the unrest – and the latter has been called on by Labour leader Sir Keir Starmer to convene urgent cross-party talks.
Earlier in the day, the Northern Ireland Executive’s five parties united to condemn the "deplorable" riots.
It is "gravely concerned by the scenes we have all witnessed on our streets", it said in a statement, including those witnessed in west Belfast on Wednesday.
Wednesday night’s violence, the most serious of several nights of unrest in loyalist communities, came amid tensions over the controversial Northern Ireland Protocol, part of the Brexit deal struck by the UK and the EU that has created what some see as a border in the Irish Sea.
There is also anger over the police announcing in March they will not prosecute anybody for attending former Sinn Fein leader Bobby Storey’s funeral last June, which allegedly breached COVID-19 restrictions on gatherings in public.
Assistant Chief Constable Jonathan Roberts said there were "upwards of 600 people present" on Wednesday evening, some of them only 13 or 14 years old.
"Young people were being encouraged to commit criminal acts by adults, who stood by clapping and encouraging the violence," Mr Roberts said.
A bus was hijacked and set on fire and a press photographer assaulted by two masked men.
Mr Roberts said a "large volume of petrol bombs" had been used, some of which were thrown into the bus.
Paramilitary involvement is an "active line of investigation" and potential "orchestration" is also being considered.
There is the potential for "imminent loss of life", while Mr Roberts said the PSNI (Police Service of Northern Ireland) is aware of other events being planned via social media.
The executive said: "While our political positions are very different on many issues, we are all united in our support for law and order."
The Stormont Assembly, which has been recalled to discuss the violence, has passed a motion calling for an immediate end to the unrest.
First minister and DUP leader Arlene Foster said: "We should all know that when politics are perceived to fail, those who fill the vacuum cause despair."
Deputy first minister and vice president of Sinn Fein, Michelle O’Neill, said it is a "miracle that, as we stand here today, no one has been killed".
Ulster Unionist leader Steve Aiken said the riots are "completely unacceptable" and condemned "organised criminal gangs bringing out children, young people and others to commit acts of destruction".
Alliance party leader and justice minister, Naomi Long, said there had been "inflammatory rhetoric with threats of renewed violence bandied around by people who claim to be trying to lead others away from the violent past".