Updated 6.30pm

A journalist was shot dead during riots in Northern Ireland in what police Friday were treating as a terrorist incident following the latest upsurge in violence to shake the troubled region.

"Lyra McKee was murdered during orchestrated violence in Creggan" in the city of Derry, also known as Londonderry, police chief Mark Hamilton said in a statement.

Republican political party Saoradh said a "republican volunteer" was trying to "defend people" from the police when the shot was fired that "accidentally" hit McKee.

McKee, 29, had earlier posted an image from the riots, accompanied by the words "Derry tonight. Absolute madness."

Images of the unrest posted on social media showed a car and van ablaze and hooded individuals throwing petrol bombs and fireworks at police vehicles.

Saoradh said "heavily armed" police went in to Creggan "to attack republicans in advance of upcoming Easter Rising Commemorations."

"The inevitable reaction to such an incursion was resistance from the youth of Creggan," added the statement.

Police chief Hamilton said "a single gunman fired shots in a residential area of the city and as a result wounded Ms McKee.

"We are treating this as a terrorist incident and we have launched a murder enquiry," he added.

Police later said that they were hunting for more than one person.

Some officials blamed Thursday's unrest on the "New IRA", a republican paramilitary group opposed to the shift towards non-violent tactics to bring about a united Ireland.

The Saoradh party denies claims it is the political wing of the New IRA.

McKee had written for The Atlantic magazine and Buzzfeed News and was named by Forbes Magazine in 2016 as one of their "30 under 30" outstanding figures, according to her literary agent Janklow & Nesbit.

Her partner Sara Canning paid tribute, saying "our hopes and dreams and all of her amazing potential was snuffed out by this single barbaric act."

'Truly senseless'

Thursday's unrest raised memories of past decades of violence in Northern Ireland.

"We cannot allow those who want to propagate violence, fear and hate to drag us back to the past," said Ireland's Prime Minister Leo Varadkar.

"This was an act of fear. This was an act of hate. This was an act of cowardice. Those who carried it out do not share the values of our nation, nor our Republic," he said.

British counterpart Theresa May said the killing was "shocking and truly senseless".

Visiting US House of Representatives Speaker Nancy Pelosi joined in a minute's silence, saying it was "especially poignant" on Good Friday.

The violence erupted in the run-up to the Easter weekend, when Republicans opposed to the British presence in Northern Ireland mark the anniversary of a 1916 uprising against British rule.

A car-bombing and the hijacking of two vans in Londonderry earlier this year were also blamed on the New IRA.

Michelle O'Neill, the deputy leader of Irish republican party Sinn Fein, condemned the killing.

"My heart goes out to the family of the young woman shot dead by so-called dissidents," she wrote on Twitter.

"This was an attack on the community, an attack on the peace process."

Arlene Foster, leader of the Democratic Union Party, which is in favour of Britain's presence in Northern Ireland, described the death as "heartbreaking news".

"Those who brought guns onto our streets in the 70s, 80s & 90s were wrong. It is equally wrong in 2019. No one wants to go back," she wrote on Twitter.

History of Troubles

The Good Friday Agreement in 1998 largely brought an end to three decades of sectarian bloodshed in Northern Ireland between republican and unionist paramilitaries, as well as British armed forces, in a period known as "the Troubles".

Some 3,500 people were killed in the conflict - many at the hands of the Irish Republican Army (IRA).

Some have expressed fears that recent attacks could be a sign that paramilitaries are seeking to exploit the current political turbulence over Northern Ireland and its border with the Republic of Ireland caused by Brexit.

Derry was the scene of one of the darkest episodes in the Troubles in 1972, known as Bloody Sunday, when British troops opened fire on a civil rights demonstration and killed 13 people. A 14th victim later died of his wounds.

A former British soldier was charged with murder last month over the killings.

Watch Lyra McKee's 2017 TED Talk here.