PENNSYLVANIA (REUTERS) - Former United States vice-president Joe Biden on Wednesday (Oct 23) slammed President Donald Trump's decision to pull US troops from north-east Syria on a "whim."

"What the hell is going on?" Mr Biden said at a rally in Scranton, Pennsylvania.

Mr Trump faces fierce criticism at home and abroad for announcing plans to pull American soldiers from Syria.

Critics say that move amounted to a green light for a Turkish military operation targeting America's Syrian Kurdish allies.

The Democratic 2020 hopeful also told supporters that the "basic bargain" in America for the middle class has been "broken."

He said Mr Trump doesn't know what it means to be part of the middle class.

"I'm not sure Donald Trump has any idea what I'm talking about when I talk about the 'longest walk'. You know, I think that the longest walk his father ever made was to drop off US$400 million in his trust account," Mr Biden said.