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

A US man has pleaded guilty to supporting a Somali Islamist militant group and encouraging attacks on the writers of cartoon show South Park.

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

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

The American Muslim convert faces up to 30 years in prison.

'Solicitation to murder'

Prosecutors said Chesser of Fairfax County in the state of Virginia also posted to an Islamist militant website the personal contact information of people who had joined an "Everybody Draw Muhammad Day" Facebook group.

"Zachary Chesser seriously endangered the lives of innocent people who will remain at risk for many years to come," US Attorney Neil MacBride said in a statement.

"His solicitation of extremists to murder US citizens also caused people throughout the country to fear speaking out - even in jest - lest they also be labelled as enemies who deserved to be killed."

US investigators said 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.

In addition, 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.

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.

The US Justice Department said that in April Chesser sought to goad Islamist militants to attack the writers of the popular US 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".

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.