A strike on an alleged drug-trafficking boat in the Pacific Ocean killed four people on Thursday, the US military said, amid growing criticism of a campaign that has taken more than 85 lives.

The strike targeted a "vessel in international waters operated by a Designated Terrorist Organisation.

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.

The strike came after a US Navy admiral told lawmakers that there was no “kill them all” order from US Defence Secretary Pete Hegseth, even as grave questions and concerns remain as Congress scrutinises an attack that killed two survivors of an initial strike on an alleged drug boat in international waters near Venezuela.

Admiral Frank “Mitch” Bradley “was very clear that he was given no such order, to give no quarter or to kill them all,” said Senator Tom Cotton, who heads the Senate Intelligence Committee, as he exited a classified briefing.

While Cotton, a Republican, defended the attack, Democrats who were also briefed and saw video of the survivors being killed questioned President Donald Trump administration’s rationale and said the boat strike was deeply concerning.

“The order was basically: Destroy the drugs, kill the 11 people on the boat,” said Adam Smith, the top Democrat on the House Armed Services Committee.

Smith, who is demanding further investigation, said the survivors were “basically two shirtless people clinging to the bow of a capsized and inoperable boat, drifting in the water.”

Lawmakers want a full accounting after The Washington Post reported that Bradley on Sept 2 ordered an attack on the survivors to comply with a directive from Hegseth to “kill everybody.”

Killing suspected drug traffickers who pose no threat of causing imminent serious injury to others would be murder under US and international law.

However, the United States has framed the attacks as a war with drug cartels, calling them armed groups.

The US Defence Department's Law of War Manual forbids attacks on combatants who are incapacitated, unconscious or shipwrecked, provided they abstain from hostilities or are not attempting to escape.

The manual cites firing upon shipwreck survivors as an example of a "clearly illegal" order that should be refused.

Hegseth said on Tuesday he had watched the first US strike in September on the suspected drug-smuggling vessel in real time, but did not see survivors in the water or the second lethal strike that he described as being carried out in the "fog of war."

But he defended Bradley's decision to carry out a follow-up strike. (Agencies)