Four men have been arrested after a car bomb attack in Northern Ireland’s Londonderry as police investigate whether the New IRA militant group was responsible.

Two men in their 20s were detained hours after the explosion on Saturday outside the city’s courthouse. Two more men, aged 34 and 42, were arrested in the city on Sunday.

No one was injured in the blast.

“Fortunately it didn’t kill anybody but clearly it was a very significant attempt to kill people here in this community,” Assistant Chief Constable Mark Hamilton said.

Mr Hamilton said the main focus of the investigation was on the New IRA – one of a small number of groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province. They have carried out sporadic attacks in recent years.

Police in Northern Ireland and European Union member Ireland have warned that a return to a hard border between the two after Brexit, complete with customs and other checks, could be a target for militant groups.

Politicians from all sides – including Sinn Fein, the former political wing of the nationalist Irish Republican Army – condemned the explosion.

Police block off the street outside the Londonderry courthouse after the blast. Photo: Getty

The Northern Ireland police force said it was given only minutes to evacuate children and hundreds of hotel guests before the explosion of what it described as a highly unstable, crude device that could have detonated at any time.

Mr Hamilton said he thought the attack marked a continuation of militants’ campaigns, rather than an escalation.

The last fatal attack involving a car bomb was carried out in 2016 by the New IRA when a prison officer was fatally injured by a device left under his van in Belfast.

About 3600 people were killed in the conflict that was fought between mainly Protestant unionists who want Northern Ireland to remain part of the United Kingdom and predominantly Catholic nationalists.

“There is no doubt that in terms of the Brexit element, there will be a section within our communities who will want to exploit that and use that to further their own objectives but I wouldn’t put that as the sole purpose,” Gary Middleton, a local Democratic Unionist Party member of Northern Ireland’s devolved government, said.

-AAP