Northern Ireland police say they have seized a van containing four mortars and arrested three men in a security operation that stopped an attack on a police station in the city of Londonderry.

About 100 homes were evacuated overnight after police stopped the van Sunday night as it drove into the city. Police arrested the driver of the van, a man riding a motorcycle behind the van and a third man during a follow-up search.

No group claimed responsibility, but Irish Republican Army splinter groups are active in the predominantly Irish Catholic city.

During its failed 1970-1997 campaign to force Northern Ireland out of the United Kingdom, the Provisional IRA developed vehicle-mounted mortar systems capable of lobbing shells into Northern Ireland's fortified police and army bases.