LONDON (AFP) - A British marine was arrested on Wednesday (Aug 24) on suspicion of terror offences as part of an investigation into Northern Ireland-related terrorism, police and security sources said.

The 30-year-old was arrested in Somerset, south-west England, on suspicion of being involved in the preparation for acts of terrorism, police said in a statement.

Contacted by AFP, security sources confirmed reports that the man is a member of the Royal Marines, an elite infantry unit, and the arrest is linked to the discovery of two arms caches in Northern Ireland earlier this year.

An armour-piercing improvised rocket and two anti-personnel mines were among the cache recovered in those raids, as well as bomb-making items.

"Today's arrest was pre-planned and intelligence-led as part of an investigation into Northern Ireland-related terrorism," the police statement said.

No armed police were involved and there was "no intelligence to suggest an immediate threat".

Searches were under way at an address and a wooded area in Devon, south-west England, and in Northern Ireland.

Bomb disposal units attended during one raid on Wednesday in Larne, a port in Northern Ireland, where one of the arms caches was found.

The Ministry of Defence said it was aware of the arrest and would assist the investigation.

The 1998 Good Friday peace accords largely ended the three decades of deadly violence in Northern Ireland, known as the Troubles, in which 3,500 people were killed.

However, the threat level for Northern Ireland-related terrorism within the province has been set at severe - the second-highest of five levels - since it was first set in 2010.

The level, determined by the MI5 domestic security service, means an attack is considered highly likely.

The threat level for Northern Ireland-related terrorism in the rest of the United Kingdom was raised one level from moderate to substantial - the third-highest level - in May this year, meaning an attack is considered a strong possibility.