Two died after shooting in north Belfast
The men, who have been named as Jackie Coulter, a prominent member of the Ulster Defence Association, and Robbie Mahood, were killed in a shooting in north Belfast on Monday.
The attack is believed to be linked to a feud between the rival Ulster Freedom Fighters/Ulster Defence Association and the Ulster Volunteer Force.
The police said the shooting victims were sitting in a blue jeep outside a bookmakers on the Crumlin Road at about 1230 BST on Monday when at least eight shots were fired at them by a gunman.
Jackie Coulter died at the scene. He was believed to have been a high-ranking member of the Ulster Defence Association with close links to convicted Ulster Freedom Fighters commander Johnny Adair.
The second man to die was named as Robbie Mahood, the brother of a prominent loyalist who was himself the target of a number of murder attempts.
Police also reported a number of other incidents in the Shankill area soon after the shooting.
Shots were fired at the Ulster Democratic Party (UDP) office and in nearby Malvern Street.
Secretary of State Peter Mandelson said the latest outbreak of violence had "nothing to do with politics" but was a result of "squalid murderous gang warfare".
"We're not going to have it. We haven't come this far in reducing tension between the two traditions only to see rival loyalist factions take away the gains we have made.
"This violence has got to end and it is got to end now. We have got to restore sanity in the situation."
Earlier on Monday afternoon, he had met security chiefs for an update on the violence.
Following the meeting it was decided to step up security by resuming army patrols in some parts of Belfast and increasing the police presence on the streets.
Army patrols on the streets of Belfast had been gradually cut back to the point where they had ceased altogether in the city as the security situation improved due to the paramilitary ceasefires.
Routine patrols ended in September 1998 but RUC Chief Constable Sir Ronnie Flanagan can seek military assistance when he deems the security situation has deteriorated.
The Progressive Unionist Party office on the Shankill Road and a taxi were also attacked by crowds in separate incidents.
Police also said a number of arrests were made and arms were seized at a house in Brookmount Street shortly after 1400 BST.
He said the shooting was a "dastardly act".
Convicted leader of the UFF Johnny Adair and Mr White went to the scene of the Crumlin Road shooting.
On Saturday, a UDA-linked festival on the Shankill Road erupted into violence when shots were fired into the Rex bar injuring seven people.
And a UFF/UDA colour party standing on the festival stage let off a volley of shots cheered on by crowds of people.
On Monday, John White called for an intermediary to be appointed to try to find a solution to the dispute between the UDA/UFF and the UVF in Belfast.