Officers are at the scene between Garrison and Belcoo in Co Fermanagh
They have closed Cashel Road from its junction with Lattone Road and the border.
Gardaí (Irish police) are assisting in the security operation by cordoning off part of Cashel Road in Co Leitrim.
The alert has been described as “very worrying” by DUP leader, Arlene Foster.
She said she has spoken to the PSNI chiefs about the operation and would “continue to liaise to ensure the safety of our community”.