Northern Ireland's top legal official says police should end their investigations into killings committed before the 1998 peace agreement, arguing they are wasteful and increasingly futile.

Wednesday's declaration by Attorney General John Harkin reignited a debate over how to promote truth and justice in a land where most of the 3,700 killings from Northern Ireland's four-decade conflict remain unsolved.

Harkin, who advises the Catholic-Protestant unity government forged by the peace pact, says all police and state-funded investigations into pre-1998 killings should end.

That drew criticism from both sides' politicians, who said relatives of the dead should never be denied the prospect of justice. British Prime Minister David Cameron also dismissed the idea.

But Northern Ireland's police commander said the pre-1998 slayings probes are weakening his force's fight against current crime.