The Provisional IRA still exists but is not on a war footing, Northern Ireland's senior police officer has said.

Individual members cooperated in shooting dead Kevin McGuigan in East Belfast but organisational structures have brought members of the outlawed organisation along the path of peace, Police Service of Northern Ireland Chief Constable George Hamilton said.

Some structures have changed, some have been dissolved but those remaining are not being used for terrorism, he said during a briefing at PSNI headquarters in Belfast.

Mr Hamilton said: "They are not on a war footing, they are not involved in paramilitary activity in the sense that they were during part of the conflict."

Sinn Féin President Gerry Adams commenting on the remarks by the PSNI chief said: "Sinn Féin stands on our record in the peace and the political processes and on our mandate.

"I note the comments by the Chief Constable today that he accepts the bona fides of the Sinn Fein leadership and our pursuit of the peace process and support for the police in the ongoing investigation into the deaths of Jock Davison and Kevin McGuigan.

"The stand of Sinn Féin on the peace process is a matter of public record. Our integrity is based on our electoral mandate and not on the views of anyone else, even a chief constable.

"I do not accept the PSNI claims about the IRA.

“The war is over. The IRA is gone and not coming back. This has been acknowledged and evidenced over the past 10 years. There is now a peaceful and democratic path to achieve republican objectives. This has the support of the vast majority of republicans and nationalists.

Earlier, Ulster Unionist leader Mike Nesbitt said Sinn Féin's credibility was "in tatters" following the shooting dead of Mr McGuigan.

Mr Nesbitt, who met the PSNI chief today, said: "The chief constable repeated the police assessment that members of the IRA took part in Wednesday's murder and that the IRA still exists, although what form it takes in 2015 is not fully clear.

"What is clear is that Sinn Féin's credibility on this issue is in tatters," he added.

Two days ago the police officer leading the investigation said one line of inquiry was that members of the provisional IRA had a role in the murder.

The two man unionist parties have since indicated that if the IRA link is established, it would have consequences for them sharing power with Sinn Féin at Stormont.

Senior Sinn Féin members have denied the IRA were involved and last night the party president Gerry Adams said the IRA left the stage in 2005.

He blamed criminals for what he called the brutal murder carried out by two gunmen in Belfast last Wednesday night.

Commenting on today's developments Minister for Foreign Affairs Charlie Flanagan said: "As these matters evolve, officials in the two Governments remain in close contact and I  expect to have an early opportunity to discuss matters directly with the Secretary of State for Northern Ireland.

"In the meantime, the focus must remain on the investigation of two heinous murders. It is essential that the PSNI be allowed continue its work, without undue political pressure and with the support of the entire community, so that those responsible are brought to justice."

A Sinn Féin delegation led by Assembly member Gerry Kelly began discussions with the chief constable in PSNI headquarters at 10.30 this morning.

Sinn Féin's Gerry Kelly said "in fairness to the chief constable he is standing by the statement made by Detective Superintendant Kevin Geddis".  

During a news conference on Thursday the officer leading the murder investigation said one line of inquiry was that Provisional IRA members were involved.  

This afternoon Gerry Kelly repeated the view that the IRA had not been active since 2005 and that Sinn Féin was fully behind the peace process and supportive of the PSNI.