WASHINGTON -- House Speaker Nancy Pelosi is suggesting that the possibility of impeachment might help Democrats win more White House co-operation in congressional investigations into whether U.S. President Donald Trump obstructed justice.

Pelosi has tapped the brakes on impeachment. But asked Wednesday by The Associated Press about some Democrats saying impeachment was the best way to reduce White House resistance, Pelosi said, "The threat of impeachment is always there."

Pelosi has cautioned fellow Democrats to see where their investigations lead. She's warned that trying to remove Trump cannot succeed without substantial Republican support.

Pelosi says Democrats favouring impeachment are expressing their views but, "They're not exerting any pressure on me."

No. 2 House Democratic leader Steny Hoyer told reporters earlier that impeachment was "the ultimate option" for forcing White House co-operation.