By Phil Stewart

WASHINGTON (Reuters) -The U.S. military carried out a new strike on Thursday against a suspected drug vessel in the Caribbean, and in what is believed to be the first such case, there were survivors among the crew, a U.S. official told Reuters.

The official, who spoke on condition of anonymity, did not offer additional details about the incident, which has not been previously reported, except to say that it was not clear that the strike had been designed to leave survivors.

The development raises new questions, including whether the U.S. military rendered aid to the survivors and whether they are now in U.S. military custody, possibly as prisoners of war.

The Pentagon, which has labeled those it has targeted in the strikes as narcoterrorists, did not immediately respond to a request for comment.

Prior to Thursday's operation, U.S. military strikes against suspected drug boats off Venezuela killed at least 27 people, raising alarms among some legal experts and Democratic lawmakers, who question whether they adhere to the laws of war.

Videos presented by the Trump administration of previous attacks showed vessels being completely destroyed, and there have been no prior accounts of survivors afterwards.

The Trump administration argues the U.S. is already engaged in a war with narcoterrorist groups from Venezuela, making the strikes legitimate. Trump administration officials say lethal strikes are necessary because traditional efforts to apprehend crew members and seize cargoes have historically failed to stem the flow of narcotics into the U.S.

The strikes come against the backdrop of a U.S. military buildup in the Caribbean that includes guided missile destroyers, F-35 fighter jets, a nuclear submarine and around 6,500 troops as President Donald Trump escalates a standoff with the Venezuelan government.

On Wednesday, Trump disclosed he had authorized the Central Intelligence Agency to conduct covert operations inside Venezuela, adding to speculation in Caracas that the United States is attempting to topple Venezuelan President Nicolas Maduro.

In a letter to the United Nations' 15-member Security Council, seen by Reuters, Venezuela's U.N. Ambassador Samuel Moncada asked for a U.N. determination that the U.S. strikes off its coast are illegal and to issue a statement backing Venezuela's sovereignty.

Less than a week ago, the Pentagon announced its counter-narcotics operations in the region would not be led by the Miami-based Southern Command, which oversees U.S. military activities in Latin America.

Instead, the Pentagon said a task force was being created that would be led by II Marine Expeditionary Force, a unit capable of rapid overseas operations that is based at Camp Lejeune in North Carolina.

That decision came as a surprise to U.S. military-watchers, since a combatant command like Southern Command would normally lead any high-profile operations.

Earlier on Thursday, Defense Secretary Pete Hegseth said the admiral who leads U.S. Southern Command will step down at the end of this year, two years ahead of schedule, in a surprise move.

The top Democrat on the Senate Armed Services Committee, Senator Jack Reed, called Admiral Alvin Holsey's unexpected resignation troubling given mounting fears of a potential U.S. confrontation with Venezuela.

"Admiral Holsey's resignation only deepens my concern that this administration is ignoring the hard-earned lessons of previous U.S. military campaigns and the advice of our most experienced warfighters," Reed said in a statement.

(Reporting by Phil Stewart; Additional reporting by Idrees Ali; Editing by Chris Reese and Lincoln Feast.)