A federal judge has ruled that former US vice president Mike Pence must testify to a grand jury about conversations he had with former president Donald Trump leading up to the attack on the Capitol, according to media reports.

In a ruling that remains under seal, the judge on Tuesday also said Mr Pence can still decline to answer questions related to January 6, 2021, CNN reported, citing unnamed sources, adding that he can still appeal the ruling.

NBC separately reported the judge’s split decision citing sources familiar with the ruling, saying it compels Mr Trump’s former vice president to appear before the federal grand jury but shields him from testifying about the attack.

The New York Times also said the ruling compels Mr Pence’s to appear.

Representatives for Special Prosecutor Jack Smith, who is leading the US Department of Justice’s investigation into Trump and his allies’ alleged efforts to overturn his 2020 election loss, could not be immediately reached for comment.

Mr Pence’s representatives also could not be immediately reached.

Mr Pence, who is exploring a challenge to Mr Trump for the Republican presidential nomination in 2024, is fighting a grand jury subpoena to secure his testimony, sources have told Reuters.

– AAP