WASHINGTON (AFP) - Jurors at Erath County courthouse in Texas on Tuesday convicted a former Marine on trial for killing the United States Navy Seal whose story inspired blockbuster American Sniper, according to court proceedings broadcast nationally.

Eddie Ray Routh, 27, was immediately sentenced to life in prison without the possibility of parole for the killings of famed sniper Chris Kyle and his friend Chad Littlefield at a Texas shooting range in February 2013.

Jurors rejected claims from Routh that he was not guilty by way of insanity.

The trial in Erath County, about 160km south-west of Dallas, attracted massive media attention across America, coinciding with the release of Clint Eastwood's controversial movie based on Mr Kyle's experiences in Iraq.

Ms Judy Littlefield, the mother of Mr Littlefield, made a brief statement late on Tuesday after the verdict.

"We have waited two years for God to give us justice on behalf of our son, and God has proved to be faithful," she said.

"We are so thrilled that we have the verdict that we have."

Routh's attorneys pleaded insanity and said their client was gripped by psychosis.

Himself a former Marine, Routh also said he suffered from post-traumatic stress disorder.

After the shootings on Feb 2, 2013, Routh took Mr Kyle's truck and was later arrested at the home of his sister.

He allegedly confessed to the killings after complaining that "people were sucking his soul and that he could smell the pigs".

During the trial, jurors heard how Mr Kyle and Mr Littlefield knew something was badly wrong minutes before Routh opened fire.

"This dude is straight up nuts," Mr Kyle wrote to Mr Littlefield in an alarmed text message.

"He's right behind me, watch my six," replied his friend, using military jargon for "watch my back".

Jurors also heard emotional testimony from Mr Kyle's widow Taya, who sobbed as she recounted their life together.

Lawyers had questioned whether Routh could receive a fair trial in the small town, given the success of American Sniper and the widely held view of Mr Kyle as a hero.

Mr Kyle, officially credited with killing 160 people during four military tours in Iraq, has been lionised in the movie, starring Bradley Cooper as the soldier.

American Sniper has so far earned more than US$320 million (S$436 million) to become the highest grossing war film in history.

Critics claim it presents a simplistic, black-and-white view of the Iraq conflict, and glosses over Mr Kyle's references to Iraqis in his memoir as "savages".