Loyalists are believed to be scouting areas of north Belfast in preparation for possible attacks.
The Irish Republican Socialist Party (IRSP) said there had been reports of ``known UFF/LVF men'' sighted in the mainly Catholic Ardoyne and Oldpark districts in recent days.
``The IRSP are cautioning all residents to be vigilant at all times following local reports that known UFF/LVF men have been spotted in the area in a beige-coloured car,'' spokesman Paul Little said.
``Given the instability of the UDA, coupled with its proven links with British intelligence murder gangs, the IRSP believe the general community is very much at risk.''
Mr Little urged Catholics in those areas to remain ``alert to the possibility of sectarian attack''.
Sinn Fein's Eoin O Broin also called for vigilance.
``For some time there has been a clear loyalist presence scouting in various districts of nationalist north Belfast, particularly the Antrim Road, Cliftonville and Ardoyne,'' the North Belfast councillor said.
``It is a matter of concern and we have made representations to unionist representatives on a number of occasions.''