Daniel Hale, 33, told a federal judge he felt compelled to leak information to a journalist out of guilt over his own participation in a program that he believed was indiscriminately killing civilians in Afghanistan far from the battlefield.

"It is wrong to kill," Hale said in a defiant statement in which he accepted responsibility for his actions but also pleaded for mercy.

"It is especially wrong to kill the defenceless."

But US District Judge Liam O'Grady told Hale he had other avenues for airing his concerns besides leaking to a journalist.

Citing the need to deter others from illegal disclosures, he imposed a punishment that was harsher than the 12- to 18-month term sought by Hale's lawyers but significantly more lenient than the longer sentence sought by prosecutors.

"You could have resigned from the military," or told "your commanders you weren't going to do this anymore," O'Grady told Hale.

The prosecution is one in a series of cases the US Justice Department has brought in recent years against current and former government officials who have disclosed classified secrets to journalists.

Attorney General Merrick Garland announced new guidelines this month to bar prosecutors from subpoenaing journalists' records in leak probes but the department has shown no signs of scaling back efforts to charge officials whom they identify as having leaked national security information.

Hale's stated rationale that he was trying to expose injustices surrounding the military's drone program has earned him support among whistleblower advocates and among critics of the government's war efforts, some of whom held supportive signs outside the courthouse and attended Tuesday's sentencing hearing.

But prosecutors painted a different portrait: Assistant US Attorney Gordon Kromberg said the impact of Hale's actions was not to contribute to a public debate over war but rather to "endanger the people doing the fight".

The Justice Department said Hale began communicating with a journalist in April 2013 while still in the Air Force.

The following February, while working as a defence contractor at the National Geospatial-Intelligence Agency, Hale printed six classified documents that were each later published.

All told, he admitted leaking roughly a dozen secret and top secret documents to a reporter in 2014 and 2015.

While court papers never specified the recipient of the leak, details about the case make it clear that the documents were given to Jeremy Scahill, a reporter at The Intercept, who used the documents as part of a series of critical reports on how the US military conducted drone strikes on foreign targets.

Australian Associated Press