U.S. senators lined up to question President Donald Trump's top intelligence officials on Tuesday over their discussion of highly sensitive war plans on a commercial messaging chat that also included a journalist, a revelation that triggered outrage among Democrats and national security experts.

Jeffrey Goldberg, editor-in-chief of The Atlantic, said in a report on Monday that he was unexpectedly invited on March 13 to an encrypted chat group on the Signal messaging app called the "Houthi PC small group" coordinating U.S. action against the Yemen rebel group over its attacks on Red Sea shipping.

National Security Council spokesperson Brian Hughes said the chat group appeared to be authentic. The White House said it was looking into how Goldberg's number was added to the thread.

Democrats - and some of Trump's fellow Republicans - called for an investigation of what appeared to be a major security breach. 

Classified and sensitive information is not supposed to be shared on commercial mobile phone apps, and unknown numbers - such as Goldberg's - should not be included.

"The administration is playing fast and loose with our nation's most classified info, and it makes all Americans less safe," Senator Mark Warner, the Democratic vice chair of the Senate Intelligence Committee said in a statement.

Members of the intelligence panel were to question two of the administration officials on the chat - Director of National Intelligence Tulsi Gabbard and CIA Director John Ratcliffe - on Tuesday when the panel holds its annual hearing on Worldwide Threats to U.S. Security.

Senator Tom Cotton, the panel's Republican chairman, said he expected the issue would come up. "I'm confident that this group chat story from yesterday's news will come up today at the hearing, and that John Ratcliffe, Tulsi Gabbard and the other leaders will have a chance to address it," Cotton said in an interview on Fox News.

"But I do hope that we keep the focus on the decisive action that the president took against these outlaw rebels in Yemen who are backed, let's remember, by Iran," Cotton said.

NO WORD ON REPERCUSSIONS

Representative Jim Himes, the top Democrat on the House of Representatives Intelligence Committee, also said he would pursue the issue when that panel holds its threats hearing on Wednesday.

"The American people deserve answers, and I plan to get some on Wednesday at the Intelligence Committee’s Worldwide Threats hearing," he said in a statement.

While some Democrats called for officials on the chat to lose their security clearances or resign, there was no immediate word on whether any official would face consequences.