A U.S. journalist was inadvertently included in a group chat in which Defense Secretary Pete Hegseth, Vice President JD Vance and other top American officials discussed upcoming strikes against Yemen's Houthi rebels, the White House confirmed Monday.

President Donald Trump announced the strikes on March 15, but in a shocking security breach, The Atlantic magazine's editor-in-chief Jeffrey Goldberg wrote that he had hours of advance notice via the group chat on Signal.

"The message thread that was reported appears to be authentic, and we are reviewing how an inadvertent number was added to the chain," National Security Council spokesman Brian Hughes said.

Trump told journalists: "I don't know anything about it. You're telling me about it for the first time." He added that "the attack was very effective" in any case.

The leak could have been highly damaging if Goldberg had publicized details of the plan in advance, but he did not do so even after the fact.

He did, however, write that Hegseth sent information on the strikes, including on "targets, weapons the U.S. would be deploying, and attack sequencing," to the group chat.

"According to the lengthy Hegseth text, the first detonations in Yemen would be felt two hours hence, at 1:45 pm eastern time," Goldberg wrote -- a timeline that was borne out on the ground in Yemen.

Goldberg said he was added to the group chat two days earlier, and received messages from other top government officials designating representatives who would work on the issue.

On March 14, a person identified as Vance expressed doubts about carrying out the strikes, saying he hated "bailing Europe out again," as countries there were more affected by Huthi attacks on shipping than the United States.

Group chat contributors identified as National Security Advisor Mike Waltz and Hegseth both sent messages arguing only Washington had the capability to carry out the strikes, with the latter official noting that he shared Vance's "loathing of European free-loading."

And a person identified as "S M" -- possibly Trump adviser Stephen Miller -- argued that "if the U.S. successfully restores freedom of navigation at great cost there needs to be some further economic gain extracted in return."

The security breach provoked outrage among Democrats, with Senate Minority Leader Chuck Schumer describing it as "one of the most stunning breaches of military intelligence I have read about in a very, very long time" and calling for a full investigation.

Senator Jack Reed also slammed the leak, saying: "The carelessness shown by President Trump's cabinet is stunning and dangerous."

The Houthi rebels, who have controlled much of Yemen for more than a decade, are part of the "axis of resistance" of pro-Iran groups staunchly opposed to Israel and the United States.

They have launched scores of drone and missile attacks at ships passing Yemen in the Red Sea and Gulf of Aden during the Gaza war, saying they were carried out in solidarity with Palestinians.

The Houthis' campaign crippled the vital route, which normally carries about 12 percent of world shipping traffic, forcing many companies into a costly detour around the tip of southern Africa.

The U.S. began targeting the Houthis in response under the previous administration of President Joe Biden, and has launched repeated rounds of strikes on Houthi targets, some with British support.

Trump has vowed to "use overwhelming lethal force until we have achieved our objective," citing the Houthis' threats against Red Sea shipping, and U.S. strikes have continued over the past 10 days.