Former U.S. Vice President Joe Biden clarified his position on a suggestion he wouldn't comply with a potential subpoena to testify at President Donald Trump's impeachment trial in the Senate.

"I have always complied with a lawful order and in my eight years as VP, my office — unlike Donald Trump and Mike Pence — cooperated with legitimate congressional oversight requests," Biden said in a Facebook post on Saturday, which was also issued as a series of tweets.

There is no legal basis for Republican subpoenas that would compel his testimony at the impeachment trial, said the 2020 presidential hopeful, who is campaigning in Iowa.