Sinn Fein has advised nationalists to be on their guard following reports that members of the UDA are patrolling sectarian interfaces in north Belfast.
The UDA men are believed to be armed.
Sinn Fein's Gerry Kelly said the news raises serious questions about the status of the loyalist ceasefire.
Meanwhile, the UDA's political wing, the Ulster Democratic Party, has called on Sinn Fein to state if the Provisional IRA is targeting loyalists in Belfast.
UDP leader Gary McMichael said police visited several loyalists last night to warn them of death threats from republican groups.
However, it was not disclosed whether the threats had come from the Provisional IRA or dissidents, such as the Real IRA.
Among those warned were loyalist prisoner, Johnny Adair and another man also being held in Maghaberry jail.