An Egyptian court has sentenced three Christian teenagers to five years in prison each for contempt of Islam after they were seen "laughing" and "mocking Muslim prayers" in a video.

A judge in the central Egyptian province of Minya also sent a fourth defendant, aged 15, to a juvenile detention centre for an indefinite period of time.

According to defence lawyer Maher Naquib, the teeners had not intended to insult Islam in the video but merely to make fun of the beheading carried out by the extremist group.

"Its unbelieavable. The judge didn't show any mercy. He handed down the maximum punishment,'' Naguib told Agence France Presse (AFP), according to Fox News.

Naguib said the judge should have just punished the teenagers with fine.

Iman Girgis, a mother of one of the convicted students, 16-year-old Moller Atef, told The Associated Press, "My son was sentenced to five years for laughing. Is that possible?" "What kind of justice is this?"

The 30-second video that was shot on a mobile phone in January 2015 shows the students pretending to pray, with one kneeling on the floor while reciting Muslim prayers while others stand behind him, laughing. Afterwards one of them is seen making a sign with his thumb to indicate the beheading of the one who is kneeling, Fox News said.

The video was reportedly filmed by the students' teacher to mock the ISIS group that beheaded Coptic Christians in Libya last year. The said teacher, who is also a Christian, was sentenced to three years in prison for insulting Islam in a separate trial.

As harsh as it may seem, the ruling is not new to religious minorities in Egypt. The activist group Egyptian Initiation for Personal Rights told AFP that 42 people were tried over the same accusation between 2011 and 2013. Twenty-seven of them were convicted.

Meanwhile, the Egyptian Commission for Rights and Freedom said in a statement that after the video's release, the teens were imprisoned for 45 days and endured "ill-treatment'' before being released pending trial

Coptic Christians make up only 10 percent of Egypt's 90-million Muslim population. They have experienced relentless violence and church attacks, according to reports.