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The Irish government says it is "very concerned" about a man caught up in the attack in Nice in which 84 were killed when a truck crashed into a crowd.

The attack coincided with Bastille Day, a national holiday in France, and thousands of people had gathered on the streets to watch a fireworks display.

Taoiseach (prime minister) Enda Kenny told RTE one Irish person was believed to have been critically injured.

The Irish embassy is in ongoing contact with the French authorities.

Live updates: Bastille Day attack in Nice

In a statement, the Irish Department of Foreign Affairs said, along with the Irish Embassy in France, they were "following up on all concerns which have been expressed to them as to the welfare of Irish citizens who may have been in the Nice area yesterday evening.

"We have particular concerns for the welfare of one Irish citizen, which we are following up urgently."

A Northern Ireland man who witnessed the attack has described the "panic" at the scene.

Paddy Mullan, from Londonderry, is in Nice on holiday with his girlfriend.

He was standing on the city's Promenade des Anglais when he saw the truck approaching.

"This lorry just mounted the kerb, across the street from us and the next thing, all you could hear was banging and shouting and screaming," he said.

Belfast man Suneil Sharma, who was nearby when the truck hit the crowds, described the aftermath as "absolute pandemonium".

Meanwhile, the Irish ambassador to France, Geraldine Byrne-Nason, told the ´óÏó´«Ã½ she was not in a position to confirm details about individual cases.

She said the embassy was working on a number of "cases of concern" but there was no confirmation from the French authorities about the nationality or identity of the victims.

Ms Byrne-Nason said Irish people in France should contact their own families in Ireland and the embassy.

Northern Ireland's first and deputy first ministers, Arlene Foster and Martin McGuinness, have issued a joint statement condemning the atrocity.

"The people who carried out this attack want to create fear and division," they said.

"Their actions have resulted in the loss of many lives of people of different nationalities. Their acts are futile and will serve only to strengthen our resolve and determination that democratic means are the only way forward."

The taoiseach, Mr Kenny, described it as an "act of madness".

"I'd like to, on behalf of the government and the Irish people, extend our sincere and deepest sympathies to President Hollande and the French people," he said.

A book of condolence for the victims of the Nice attack is due to be opened at Belfast City Hall on Monday.

Belfast City Council said the City Hall will be illuminated in the colours of the French flag, as a mark of solidarity with the French people on Friday evening.

The British and Irish embassies in France have both issued telephone helpline numbers for any of their citizens who have been affected by the Nice attack

The British embassy's 24-hour number is: +33 144513100.

The Irish embassy can be contacted on: + 33 144176700.