Defence lawyers have vowed to appeal the convictions of four former Blackwater security guards after a federal judge handed down lengthy prison terms for their roles in a 2007 shooting of unarmed civilians in Iraq.

They identified several issues as likely to form the basis of an appeal, including vindictive prosecution and whether State Department contractors could be charged under a federal law which covers the overseas crimes of Defence Department civilian employees.

The move comes after US District Judge Royce Lamberth sentenced former guard Nicholas Slatten to life in prison and three others to 30-year terms for their roles in the shootings that killed 14 Iraqi civilians and wounded 17 others in Baghdad's Nisoor Square.

Slatten, who witnesses said was the first to fire shots in the melee, was sentenced to life after being convicted last October of first-degree murder. The three other guards - Paul Slough, Evan Liberty and Dustin Heard - were each sentenced to 30 years and one day in prison for offences including manslaughter, attempted manslaughter and using firearms while committing a felony.

The incident strained US-Iraq relations and caused an international uproar over the use of private security guards in a war zone.

Judge Lamberth announced the sentences after a day-long hearing at which defence lawyers had argued for leniency and presented character witnesses for their clients. At the same time, prosecutors asked that those sentences - the minimums mandatory under the law - be made even harsher. He rejected both requests.

Appearing in court wearing leg shackles and prison garb, the former contractors insisted they were innocent.

"The verdict is wrong, you know that I am innocent, sir," Slatten told the judge.

"I feel utterly betrayed by the same government I served honourably," said Slough.

But Judge Lamberth said he fully agreed with the jury's guilty verdicts and praised the Justice Department and the FBI for investigating the shooting and putting the truth "out there for the world to see".

Nearly 100 friends and relatives packed the courtroom to show support for the men, with many openly weeping throughout the proceedings. Several came to the podium, some choking back tears, to speak glowingly of the men they knew as role models and patriots who only wanted to help serve their country.