SYDNEY (REUTERS) - A Chinese official's tweet of an image of an Australian soldier that sparked a furious reaction from Canberra was amplified across social media by unusual accounts, of which half were likely fake, an Israeli cyber-security firm and Australian experts said.

The digitally altered image of an Australian soldier holding a bloodied knife to the throat of an Afghan child was tweeted by China's foreign ministry spokesman Zhao Lijian on Monday (Nov 30).

Twitter declined Australia's request to remove the tweet.

The Chinese embassy in Canberra told ABC television on Friday that Prime Minister Scott Morrison's demand for an apology drew more attention to an investigation into war crimes by Australian soldiers in Afghanistan.

Cyabra, an Israeli cyber-security firm, said it found evidence of an orchestrated campaign to promote Mr Zhao's tweet.

Cyabra said it had found 57.5 per cent of accounts that engaged with Mr Zhao's tweet were fake, and "evidence of a largely orchestrated disinformation campaign" to amplify its message.

The firm did not give any details about who was behind the campaign.

Cyabra said it analysed 1,344 profiles and found a large number were created in November and used once - to retweet Mr Zhao's tweet.

China called Cyabra's statement "unwarranted". "This is a classic example of spreading false information. Twitter has its own rules managing tweets," the Foreign Ministry said late on Friday in response to Reuters questions.

The Queensland University of Technology's Tim Graham analysed 10,000 replies to Mr Zhao's tweet.

Accounts originating in China were the most active, he said, and 8 per cent of replies were from accounts created on the day, or in the 24 hours prior. Many contained duplicated text.

"When not tweeting about Afghan children, they were tweeting about Hong Kong," Dr Graham told Reuters in an interview. "If there's enough of them, those irregularities suggest they were set up for a particular campaign."

Some of the accounts had already been identified by Dr Graham in a data set of 37,000 Chinese accounts targeting Australia since June, he said.

Ms Ariel Bogle, a researcher at the Australian Strategic Policy Institute, said she had also noticed "unusual behaviour" by Twitter accounts retweeting or liking Mr Zhao's tweet.

"There was a spike in accounts created on Nov 30 and Dec 1," she told Reuters, adding it was too early to determine if it was coordinated inauthentic behaviour or patriotic individuals.

Many of the new accounts followed only Mr Zhao, plus one or two other accounts, she said. A third of accounts liking Mr Zhao's tweet had zero followers, the Australian Strategic Policy Institute noted.

Earlier this year, Twitter said it had removed 23,750 accounts spreading geopolitical narratives favourable to the Chinese Communist Party, and another 150,000 accounts designed to amplify these messages.

A Twitter spokesman said the company remains vigilant, but the Cyabra findings "don't hold up to scrutiny" because it relied only on publicly available data.

A Cyabra spokesman said its founders are information warfare experts with Israeli military backgrounds, and the United States State Department was among its clients.