Image caption Chesser, shown near the White House, promoted violence on his web site, prosecutors said

A US man convicted of encouraging attacks on the writers of cartoon show South Park has been sentenced to 25 years in prison.

Prosecutors said Zachary Adam Chesser, 20, was outraged by the cartoon's perceived mockery of the prophet Muhammad.

They said he sought twice to travel to Somalia to join al-Shabab, which the US designates as a terrorist group.

The American Muslim convert pleaded guilty to federal charges in October.

'Incitement to violence'

According to the US justice department, Chesser of Fairfax County in the state of Virginia, admitted he hoped to encourage violent Islamists to attack the writers of the popular satirical cartoon South Park in retaliation for the show's depiction of Muhammad wearing a bear suit.

Prosecutors said he urged online readers to "pay them a visit".

And in addition, Chesser admitted posting on an Islamist militant website the personal contact information of people who had joined an "Everybody Draw Muhammad Day" Facebook group.

"Zachary Chesser attempted to provide material support to a foreign terrorist organization and used the Internet to incite violence," Assistant Attorney General Kris said in a statement. "Today he is being held accountable for his actions."

Assistant Attorney General Neil H MacBride said Mr Chesser advocated "the murder of US citizens for engaging in free speech about his religion".

"His actions caused people throughout the country to fear speaking out - even in jest - to avoid being labelled as enemies who deserved to be killed," he said. "The fact that a young man from Northern Virginia could support such violence and terror is a sobering reminder of the serious threat that homegrown jihadists pose to this country."

In addition, US investigators say Chesser was a follower of radical US-born cleric Anwar al-Awlaki, who is believed to be in Yemen with al-Qaeda in the Arabian Peninsula.

Prosecutors said Chesser posted messages online from Mr al-Awlaki in which he called for violent attacks, and posted links to internet forums giving advice on how to plan them.

Chesser pleaded guilty to urging people to plant suspicious packages in public places in order to "densensitise" police so a real bomb would escape notice.

His efforts to travel to Somalia to join al-Shabab failed when his wife was unable to obtain a passport and when he was prohibited from boarding a flight with his infant son, prosecutors said.

Al-Shabab wants to impose a strict version of Sharia law in Somalia, where they control most of the south and centre of the country. The fragile UN-backed government only controls parts of the capital, Mogadishu.

Chesser pleaded guilty to two counts of communicating threats and soliciting crimes of violence, as well as to supporting an al-Qaeda linked terrorist group.