Almost a third of pro-Donald Trump tweets around the first US presidential debate were not sent by humans but by online bots compared to just over a fifth of pro-Hillary Clinton posts, a new Oxford University study says.

The Press Trust of India (PTI) reports that more than four times as many tweets were made by automated accounts in favour of Republican candidate Trump around the first US presidential debate on Sept 26 as by those backing his Democratic rival Clinton, the BBC reported on the eve of the third and final encounter between the two in Las Vegas, as quoted by PTI.

The bots exaggerated support for the 70-year-old Republican presidential hopeful, the study suggests, but Trump would still have won a higher number of supportive tweets even if they had not.

The authors warn such software has the capacity to "manipulate public opinion" and "muddy political issues".

The investigation was led by Prof Philip Howard, from the University of Oxford, and is part of a wider project exploring "computational propaganda".

It covered tweets posted on Sept 26, the day of the first debate, plus the three days afterwards, and relied on popular hashtags linked to the event.

The results suggested that 32.7 per cent of such pro-Trump tweets had been posted by bots and 22.3 per cent of such pro-Clinton ones.

In total, that represented a total of 576,178 tweets benefiting the Republican nominee and 136,639 in support of the Democratic one.

"On the balance of probabilities, if you pulled out a heavily automated account the odds are four to one that you'll find it's a bot tweeting in favour of Trump," Howard said.

There is no suggestion, however, that bots were generated by either of the official Presidential campaign groups, the report said.

Looking wider - to accounts that tweeted neutral hashtags or a mix of different kinds - the study suggested that 23 per cent of all the tweets were driven by bots. -- BERNAMA