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Ex-PM also claimed the West is "doomed unless conservatives win again".
He made the comments as a New York judge gave the go-ahead for his hush-money trial to begin next month.
After Biden denounced his comments, Trump called him "uninformable."
“Can you imagine? A former president of the United States saying that?” the president said of Trump encouraging Russia to attack US allies.
The hearing is reportedly being held at a Sensitive Compartmented Information Facility in Fort Pierce, Florida.
The US president also took a shot at Donald Trump during his first video on the social network.
Unpicking predictions about the famously unpredictable former – and possibly future – US president.
Lawyers described the US president as an "elderly man with a poor memory" in a report into his handling of classified documents.
The report concludes a yearlong saga after classified documents were found at Biden's home in Delaware.
Both Russia and China have been accused of interfering in other countries themselves.