Two Danish brothers originally from Somalia will spend four weeks in pre-trial detention following their arrest by Denmark's security service on suspicion of plotting a terror attack.

The security service says the older brother, who is 23, is also suspected of having received terror training from the Somali militant group al-Shabab.

He was arrested late yesterday at Copenhagen's international airport as he arrived by plane from an undisclosed location. At the same time, his 18-year-old brother was arrested in a city (Aarhus) in western Denmark.

The head of the security service says authorities cannot say with certainty that a terrorist act was imminent, but felt that it was necessary to intervene and arrest them at this time to be able to thwart the plans.

Both men pleaded innocent at a custody hearing today.

Al-Shabab relies on several hundred foreign fighters - some with experience in Iraq, Afghanistan and Pakistan. It joined al-Qaida earlier this year, and seeks to recruit new soldiers from Somali communities overseas.