A man described by a US judge as a leader of a group who plotted to travel to Syria to fight for the Islamic State group has been jailed for 35 years.

Guled Omar, 22, was one of three Somali-American men sentenced on terror-related charges by the court in the state of Minnesota on Wednesday.

The other two, Abdirahman Daud and Mohamed Farah, also 22, were each jailed for 30 years.

Six other defendants have already been sentenced to terms of up to 15 years.

Omar, Daud and Farah were convicted by a jury in June of conspiring to provide material support to a foreign terrorist organisation and conspiracy to commit murder outside the US.

As he awaited his sentence, Omar told the judge that he understood the gravity of his conviction.

I always had energy for justice as a young man but I lost my way," he told the court.

But US District Judge Michael Davis described the group of nine as a "terrorist cell" and said he didn't believe Omar's words of contrition.

Prosecutor Andrew Winter said Omar's crying was crocodile tears.

"Only when backed into a corner, does he attempt to offer false contrition," he said.

"You can't fix manipulative. You can't fix deceitful. And you can't fix Guled Omar."

Prosecutors said the plot had involved a group of friends who planned to travel to Syria and fight alongside IS.

The Minneapolis area is home to the largest concentration of Somali immigrants in the US.

Since 2007, at least 22 Somali men have travelled oversees to support Islamic State or Al-Shabab, a separate militant group based in Somalia.