US President Donald Trump has slammed whistleblower Chelsea Manning as an "ungrateful traitor" for criticising Barack Obama.

Manning, whose 35-year sentence for leaking classified information to WikiLeaks was commuted by Barack Obama as one of his last acts as president, now has a column in The Guardian.

In her first column for the newspaper, Manning attacked Obama as a weak leader with "few permanent accomplishments".

"The one simple lesson to draw from President Obama's legacy: do not start off with a compromise," Manning wrote. "They won't meet you in the middle. Instead, what we need is an unapologetic progressive leader."

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Trump, who has also been critical of Obama and his administration, said Manning's criticism of the former president was "terrible."

"Ungrateful TRAITOR Chelsea Manning, who should have never been released from prison, is now calling President Obama a weak leader. Terrible," he wrote.

Manning, a transgender woman, also criticised Obama for his response to the Pulse nightclub attack in Orlando, which targeted gay and lesbian patrons on Latin night.

"In the aftermath of the deadly shooting at the Pulse nightclub in Orlando that took the lives of nearly 50 queer and brown people, it took Obama over 300 words of his speech to acknowledge the queer community, and even then, as an abstract acronym," she said.

President Barack Obama has commuted the prison sentence of Chelsea Manning, the former US military intelligence analyst who is serving 35 years in prison for passing classified files to WikiLeaks, the White House says. Credit: CNN

Australian Julian Assange had said that he would be extradited to the US from the Ecuadorean embassy in London if Manning was released. However, he has flip flopped on whether he will honour his promise.

Manning is set to be released from prison in May.

Former US President Barack Obama has been criticised by Chelsea Manning as being weak. Photo / AP
Former US President Barack Obama has been criticised by Chelsea Manning as being weak. Photo / AP

US Army Private Chelsea Manning's 35-year sentence for leaking classified information to the anti-secrecy website WikiLeaks was commuted by President Barack Obama to about seven years, including the time she spent locked up before she was convicted in 2013.

Here's a look at the key elements of the case:

WHAT WAS MANNING CONVICTED OF?

A judge convicted Manning, then known as Bradley, on July 30, 2013, of 20 counts, including six Espionage Act violations, theft and computer fraud. She was sentenced to 35 years out of a possible maximum of 90. She was acquitted of the most serious charge of aiding the enemy, which carries a possible life sentence.

WHAT DID MANNING DO?

The now 29-year-old native of Crescent, Oklahoma, leaked more than 700,000 classified Iraq and Afghanistan war logs and diplomatic cables in 2010 while working as an intelligence analyst in Baghdad. Manning also leaked a 2007 video clip of a US helicopter crew killing at least nine men, including a Reuters news photographer and his driver. The Pentagon concluded the troops acted appropriately, having mistaken the camera equipment for weapons.

WHAT HARM DID THE PROSECUTORS' EVIDENCE SHOW?

Government witnesses testified the leaks endangered people who were named as information sources, prompting the State Department to help some of them move, even to other countries, for their safety. Several ambassadors were recalled, expelled or reassigned because of embarrassing disclosures.

Prosecutors showed that al-Qaida in the Arabian Peninsula used material from the Apache helicopter attack in a propaganda video. Al-Qaida leader Osama bin Laden obtained and presumably read some of the leaked documents, the evidence showed.

WHAT WERE THE DEFENSE'S PRINCIPAL ARGUMENTS?

The defense produced evidence that the Army disregarded Manning's emotional turmoil over her gender identity and isolation in a military that barred homosexuals from serving openly. The day after she was sentenced, Manning announced in a statement that she was a woman named Chelsea and demanded hormone therapy, which the Army eventually approved.

WHY DID SHE DO IT?

Manning said she leaked the material to expose the US military's "bloodlust" and disregard for human life, and what she considered American diplomatic duplicity. She said she chose information she believed would not harm the United States. After her conviction, she apologised for unintentionally causing harm, but not for revealing US secrets.