A Finnish court remanded in custody Iraqi twin brothers suspected of killing 11 people in a massacre by the Islamic State group in Iraq in 2014.

'The Tampere district court has remanded in custody, upon the National Bureau of Investigation's (NBI) request, two men who are suspected of 11 murders with terrorist intent,' the NBI said in a statement after the court's hearing.

The two men, both 23, pleaded not guilty on suspicion of shooting dead 11 unarmed captives during a massacre in the Iraqi city of Tikrit in June 2014.

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Finnish authorities escort a 23-year-old suspect (centre) into the court room in Tampere, on December 11, 2015

One of the ISIS fighters shows piles of victims waiting to be gunned down, somewhere near the city of Tikrit

In July 2015, IS released footage of the massacre in which it executed hundreds of mostly-Shiite military recruits captured at the Speicher military base in Tikrit, hometown of late Iraqi dictator Saddam Hussein.

The highest estimates put the number of executed cadets at 1,700.

Raty said that the IS video of the massacre was a central piece of evidence in the investigation.

'The nature of the crime is visible in the video... They (the suspects) were not hooded,' he said.

Raty told Finnish public broadcaster YLE earlier that 'the victims were lying on the ground and they were shot one by one.'

Two propaganda videos were released with the jihadi group showing off how they rounded up the recruits and brutally executed them in cold blood

In July 2015, IS released footage of the massacre in which it executed hundreds of mostly-Shiite military recruits captured at the Speicher military base in Tikrit, hometown of late Iraqi dictator Saddam Hussein

The armed militant group forced their victims to form a human chain and led them off to be killed

Dozens of people were killed as the gunmen forced their victims to wait in line to be executed

Other asylum seekers at the centre told tabloid Ilta-Sanomat they had sensed something 'odd' about the twins' behaviour

He would not comment on why the Iraqi brothers had come to Finland in September, but Finnish media reported they had been staying at a reception centre for asylum seekers in the southwestern town of Forssa where they were arrested on Tuesday.

Other asylum seekers at the centre told tabloid Ilta-Sanomat they had sensed something 'odd' about the twins' behaviour.

'I saw them sit alone and they would not talk to anyone. I knew they were hiding something,' a 38-year-old asylum seeker and lawyer who gave his name as Omar Mohamedi told the paper.

Another man claimed the twins, who are Sunni, viewed Shiite Muslims with disdain.

The horrific massacre was carried out at 9:30am on 12 June 2014 and was released as a vile propaganda video

Finland's Interior Minister Petteri Orpo reiterated an earlier estimate by the country's Security Police that around 300 people in the country are known to have connections to 'terrorist' groups abroad.

The NBI said the arrests were the result of close cooperation between the Security Police and local police forces who have been conducting asylum interviews with newly-arrived refugees.

'The crucial intelligence has come from within the country but we do seek to cooperate with foreign authorities on this case,' Raty said, refusing to confirm whether police had been tipped off by an asylum seeker.

The prosecutor has until April 8 to press charges against the twins.

An Iraqi soldier guards the site of a mass grave at the Speicher military base in Tikrit, where hundreds of Shiite recruits were executed by Islamic State jihadists in 2014