Defense Secretary Pete Hegseth and President Donald Trump's administration have especially come under fire over an incident in early September in which US forces targeted the wreckage of a vessel that had already been hit, killing two survivors.

A senior Democratic lawmaker who saw footage of that incident on Thursday said it showed a US attack on "shipwrecked sailors," while others have described it as a possible war crime.

The latest strike targeted a "vessel in international waters operated by a Designated Terrorist Organization. Intelligence confirmed that the vessel was carrying illicit narcotics and transiting along a known narco-trafficking route in the Eastern Pacific," US Southern Command said in a post on X.

"Four male narco-terrorists aboard the vessel were killed," said the post, which included a video showing a multi-engine boat speeding across the water before being hit by a blast that left the vessel engulfed in flames.

Earlier in the day, lawmakers attended a classified briefing on Capitol Hill in which they were shown extended video footage of the strike, only a brief part of which has been publicly released.

The footage showed "the United States military attacking shipwrecked sailors -- bad guys, bad guys -- but attacking shipwrecked sailors," Representative Jim Himes, the top Democrat on the House Intelligence Committee, told journalists.

- No 'imminent threat' -

Himes described it as "one of the most troubling things I've seen in my time in public service."

"You have two individuals in clear distress without any means of locomotion, with a destroyed vessel, who were killed by the United States," he added.

Republican Representative Don Bacon meanwhile said on CNN that "these two people were trying to survive and our...rules of war would not allow us to kill survivors."

"The rules are they have to pose an imminent threat. And I think we could say they did not pose an imminent threat to our country," Bacon said.

Republican Senator Tom Cotton, another attendee of the briefing, defended the military action, saying "the first strike, the second strike, and the third and the fourth strike on September 2 were entirely lawful and needful, and they were exactly what we'd expect our military commanders to do."

"I saw two survivors trying to flip a boat loaded with drugs bound for the United States back over so they could stay in the fight," he said of the footage.

However, both the White House and Pentagon have sought to distance Hegseth from the decision to strike the survivors, instead pinning the blame on Admiral Frank Bradley, who directly oversaw the operation.

Himes said Bradley told lawmakers during the briefing that Hegseth did not order that all the boat's crew be killed, but Bacon said the Pentagon chief is ultimately responsible because "he's the secretary of defense."

Trump's administration insists it is effectively at war with alleged "narco-terrorists," and the president has deployed the world's biggest aircraft carrier and an array of other military assets to the Caribbean, insisting they are there for counter-narcotics operations.

Regional tensions have flared as a result of the strikes and the military buildup, with Venezuela's leftist leader Nicolas Maduro accusing Washington of using drug trafficking as a pretext for "imposing regime change" in Caracas.