WASHINGTON (AFP) - President Donald Trump has taken a swing on Twitter at one of his favourite punching bags, Democrat Hillary Clinton - and his former presidential rival has hit back with her own zinger.

The trouble started on Wednesday (May 31) when Clinton raised questions over whether Trump's campaign was linked to the alleged Russian meddling in the 2016 US presidential election.

Speaking at a technology conference in California, Clinton said Russia's government "could not have known how best to weaponise that information unless they had been guided" by Americans with polling data.

Clinton was referring to the hacking of her campaign's e-mails, which she has argued was partly to blame for her losing to the real estate mogul.

"I'm leaning Trump," she said on Wednesday of the connections. "I think it's pretty hard not to."

Trump was not buying it.

"Crooked Hillary Clinton now blames everybody but herself, refuses to say she was a terrible candidate. Hits Facebook & even Dems & DNC," he wrote on Twitter.

Clinton has also placed partial blame on fake news about her being widely spread on Facebook and now ex-FBI director James Comey re-opening his probe into her misuse of government e-mail.

She took to Twitter to fire back at Trump.

"People in covfefe houses shouldn't throw covfefe," she wrote in a brief message.

Trump used the non-existent word in an incomplete sentence tweeted early on Wednesday. The tweet remained online for some 5.5 hours, sparking worldwide ridicule.

The message was finally deleted, and Trump joked about it in a subsequent tweet.

Spokesman Sean Spicer was asked about the cryptic word.

"The president and a small group of people know exactly what he meant," Spicer told incredulous reporters at a news briefing. He offered no further explanation.