Nancy Pelosi, Speaker of the US House of Representatives, has written a letter to President Donald Trump inviting him to give his State of the Union speech on 4 February amid the partisan impeachment battle, BBC News reports.

The letter sent by Pelosi on Friday begins with a pointed reminder to Trump of the "separation of powers" enshrined in the US Constitution.

The three branches - the judicial, executive, and legislative - she said are "co-equal branches acting as checks on each other."

"In the spirit of respecting our Constitution, I invite you to deliver your State of the Union Address," she wrote, adding: "Thank you for your attention to this matter."

White House spokesman Hogan Gidley said that Donald Trump had accepted the invitation to speak.