WASHINGTON (Reuters) - Roger Stone, a longtime ally of President Donald Trump, refused to respond to one line of questioning from members of the U.S. House of Representatives Intelligence Committee on Tuesday and could face a subpoena to return and answer them, the panel's top Democrat said.

"Hopefully, he will cooperate in the future. If not, it will be necessary to subpoena him to bring him back to answer those important questions," Representative Adam Schiff told reporters after Stone completed a three-hour meeting with committee members.

(Reporting by Patricia Zengerle; Editing by Jonathan Oatis)