The main Belfast to Dublin railway line remains closed between Newry and Dundalk due to security alerts.
Fires engulfed a number of businesses in the city
The dissident republican Real IRA said on Friday it had left two devices on the line.
The group also claimed responsibility for the firebomb attacks on stores in Newry earlier this week.
Firebombs destroyed JJB Sports and CarpetRight stores in the town whilst a TK Maxx store and MFI outlet were among those badly damaged on Wednesday.
The attacks are estimated to have caused damage worth hundreds of thousands of pounds.
Newry's SDLP mayor Michael Carr said: "The Real IRA should examine their motives.
"Their cause is not a blow for Ireland, but a blow against their own communities."
Ulster Unionist assembly member Danny Kennedy urged the government to ensure that all resources are given to the PSNI to "enable them to tackle this threat and root out those responsible".