DUBLIN (Reuters) - Irish police seized a substantial quantity of firearms and a suspected explosive device near the border with Northern Ireland on Friday as part of an investigation into the activities of militant Irish nationalist groups there.

The searches took place near the County Louth village of Omeath, which lies on a small peninsula separating part of the border between European Union-member Ireland and the British province of Northern Ireland.

The currently seamless border will become the United Kingdom’s only land frontier with the EU when it leaves the bloc, scheduled for the end of March.

Police on both sides have warned that any hardening of the border as a result of Brexit, with the potential return of customs posts, could be a target for the small number of militant groups still active in Northern Ireland after a 1998 peace deal ended three decades of violence in the province.

“During the searches to date Gardaí (Irish police) have recovered a substantial quantity of ammunition of varied calibre along with a mortar tube (pending examination by Garda ballistics experts),” the police said in a statement.

The recovery of the ammunition was part of ongoing police investigations targeting militant groups based in the area. The army’s bomb disposal unit was assisting at the scene, the statement added.

A militant group calling itself the “IRA” said this week it was responsible for a car bomb which detonated outside a court house in the Northern Irish city of Londonderry in January, highlighting the threat still posed by groups opposed to the peace agreement.

No one was injured in the blast, which the group said was not linked to Brexit.