The cross-border rail link between Belfast and Dublin has been closed after several explosions were heard in the vicinity of the line.
A security operation was mounted at first light to try to pinpoint the seat of the explosions, close to the border town of Newry in County Down.
Rail services have been suspended as thousands of rugby fans from Northern Ireland prepare to travel to Dublin for the Ireland against France Six Nations' match at Lansdowne Road.
Northern Ireland Railways, which normally lays on extra trains to get fans to international rugby matches, says they should make their own way to Dublin as it cannot guarantee to provide a substitute bus service around the blast scene.
A spokesman for the Royal Ulster Constabulary says: "A number of explosions have been heard in the vicinity of the railway line at Newry. The cause is under investigation at the moment".
Dissident republicans have mounted a campaign against the railway line recently.
It only fully reopened a few days ago after a five-day security operation on the line outside Newry. This resulted in the discovery of a bomb crater beside the track, caused by a device containing an estimated 100lb of home-made explosives.
Only yesterday, services were disrupted for a time following a bogus warning of a device further up the track between Moira and Lurgan, County Armagh.