CHICAGO (AFP) - An American man who admitted to scouting targets ahead of the deadly 2008 Mumbai attacks and then cooperated with US authorities to avoid execution was sentenced on Thursday to 35 years in prison.

David Coleman Headley, 52, pleaded guilty in 2010 to 12 charges related to the carnage in Mumbai and a second plot to attack a Danish newspaper that sparked outrage over its publication of cartoons of the Prophet Mohammed.

He convinced US federal prosecutors to let him live after he was caught on tape plotting the Danish attack by telling them all he had learned in seven years of working with Pakistani militants.

Former US attorney Patrick Fitzgerald, who brokered the plea deal, said the information Headley provided "saved lives" as he urged Judge Harry Leinenweber to be lenient rather than imposing the maximum sentence of life in prison.

In handing down the 35-year prison sentence, Judge Leinenweber however told Headley he would much rather impose the death penalty, saying: "That's what you deserve."

Heavily-armed militants rampaged through Mumbai in Nov 2008, killing 166 people and wounding hundreds more over nearly three days of carnage in a prolonged assault on the Indian financial capital.

In a plot that reads like a spy thriller, Headley spent two years casing Mumbai, even taking boat tours around the city's harbour to find landing sites for the attackers and befriending Bollywood stars as part of his cover.

He was so eager to attack Denmark's Jyllands-Posten newspaper and kill its cartoonist that he began working seriously on that plot two months before the Mumbai attack.

He also had Bollywood and one of India's most sacred Hindu temples in his sights as he began plotting a second India attack during a March 2009 surveillance trip.

Judge Leinenweber said that while the damage that Headley had caused was "unfathomable," he decided to fulfill the government's request because the recommended 30 to 35 years "is not a light sentence" given Headley's age.

"I'm hopeful it will keep Mr. Headley under lock and key for the rest of his natural life," the judge said.

India objected after US prosecutors agreed not to extradite Headley in exchange for his cooperation after his Oct 2009 arrest in Chicago's O'Hare airport as he was set to board a flight to Pakistan.

Acting US attorney Gary Schapiro said he hoped the citizens of India would understand that the information Headley provided helped prevent other attacks.

"The entire point of this exercise is to encourage future witnesses to also help us bring terrorists to justice," Mr Schapiro told reporters after the sentencing.

Ms Lisa Monaco, assistant attorney general for national security, called the sentence "an important milestone in our continuing efforts to hold accountable those responsible for the Mumbai terrorist attacks and to achieve justice for the victims".

The Washington-born son of a former Pakistani diplomat and American woman, Headley's Western appearance and US passport helped him slip under the radar.

Headley, who changed his name so he could hide his Pakistani heritage, joined Lashkar-e-Taiba (LeT) - the group India blames for the attacks - in 2002, attending terror training camps five times in three years.

US prosecutors have kept most of the information provided by Headley under wraps but have said he provided details about dozens of potential targets in India and Denmark that were under surveillance.

Co-conspirator Tahawwur Hussain Rana, 52, was sentenced to 14 years in prison last week for letting Headley use his Chicago-based immigration firm as a cover while working on the Denmark plot.

There is no parole for federal crimes. Prisoners must serve at least 85 per cent of their sentences.