The Trump administration has yet to provide underlying evidence to lawmakers proving that alleged drug-smuggling boats targeted by the U.S. military in a series of fatal strikes were in fact carrying narcotics, according to two U.S. officials familiar with the matter.

As bipartisan frustration with the strikes mounts, the Republican-controlled Senate on Wednesday voted down a war powers resolution that would have required the president to seek authorization from Congress before further military strikes on the cartels.

The military has carried out at least four strikes on boats that the White House said were carrying drugs, including three it said originated from Venezuela. It said 21 people were killed in the strikes.

The officials, who were not authorized to comment publicly about the matter and spoke on the condition of anonymity, said the administration has only pointed to unclassified video clips of the strikes posted on social media by President Donald Trump and Defense Secretary Pete Hegseth and has yet to produce "hard evidence" that the vessels were carrying drugs.

The administration has not explained why it has blown up vessels in some cases, while carrying out the typical practice of stopping boats and seizing drugs at other times, one of the officials said.

The Republican administration, in a retroactive memo justifying one of the strikes last month, declared drug cartels to be "unlawful combatants" and said the United States is now in an "armed conflict" with them.

The declaration has raised stark questions about how Trump intends to use his war powers. It also has been perceived by several senators as pursuing a new legal framework to carry out lethal action and has raised questions about the role of Congress in authorizing any such action.

Asked about the lack of underlying evidence provided to Congress, the Pentagon on Wednesday pointed to videos of the strikes, which do not confirm the presence of drugs.

The Pentagon also noted public statements by Hegseth, including a social media post following the latest fatal strike in which he said, "Our intelligence, without a doubt, confirmed that this vessel was trafficking narcotics, the people onboard were narco-terrorists, and they were operating on a known narco-trafficking transit route."

Lawmakers have expressed frustration that the administration is offering little detail about how it came to decide the U.S. is in armed conflict with cartels or even detailing which criminal organizations it claims as "unlawful combatants."

Independent Sen. Angus King of Maine said Wednesday that he and other members of the Senate Armed Services Committee, in a classified briefing this week, were denied access to the Pentagon`s legal opinion about whether the boat strikes adhered to U.S. law.

His comments came at a confirmation hearing for Joshua Simmons, a top legal adviser to Secretary of State Marco Rubio, to be the CIA`s next general counsel. At the hearing, Simmons refused to say whether he had partaken in any deliberations over the targeting of cartels in the Caribbean, saying any legal advice he gave Rubio or other U.S. officials would`ve been confidential.

Attorney General Pam Bondi was pressed at a Senate hearing Tuesday about what advice she`s provided Trump to legally justify the strikes. She said, "I`m not going to discuss any legal advice that my department may or may not have given or issued at the direction of the president."

A White House official suggested that lawmakers were being disingenuous with their criticism and that the Trump administration has been "much more forthcoming" with the legal rationale than Democratic President Barack Obama`s administration was when it carried out strikes targeting militants in the Middle East.

The official, who was not authorized to comment publicly and spoke on the condition of anonymity, said Pentagon officials have held six separate classified briefings to Congress on the operations.

Trump administration officials have argued that the strikes are necessary acts of self-defense as cartels funnel drugs into the United States that they say are leading to thousands of U.S. deaths. While Venezuela produces cocaine, the bulk of it is sent to Europe.

Trump has largely bypassed traditional interagency processes in formulating his strategy to carry out strikes against drug cartels, according to the U.S. officials and a person familiar with the matter who spoke on condition of anonymity to discuss the sensitive matter.

A small group of top administration officials - including Rubio, Deputy Secretary of State Christopher Landau and Trump aide Stephen Miller - has driven the push to carry out the fatal strikes, officials said.

Rubio, dating back to his days in the Senate, has advocated for taking a harder line on Venezuelan President Nicolás Maduro.

During Trump`s first term, Maduro was indicted on U.S. federal drug charges, including narcoterrorism and conspiracy to import cocaine. This year, the Justice Department doubled a reward for information leading to Maduro`s arrest to $50 million, accusing him of being "one of the largest narco-traffickers in the world."

Trump has focused attention on the Venezuelan gang Tren de Aragua, which he claims is serving as "front" for Maduro, and said members of the gang were in the first boat targeted last month. No details on alleged affiliations have been released in the three other strikes.

Maduro was sworn in for a third six-year term in January despite credible evidence that he lost last year`s election. The U.S. government, along with several other Western nations, does not recognize Maduro`s claim to victory and instead points to tally sheets collected by the opposition coalition showing that its candidate, Edmundo González, won by more than a two-to-one margin.

Early in his term, however, Trump dispatched special envoy Richard Grenell to Caracas to meet with Maduro. Six Americans who had been detained in Venezuela were freed by Maduro`s government during Grenell`s visit.

But diplomatic efforts with Caracas have been largely paused in recent months, with Grenell mostly sidelined, said the person familiar with the matter and a congressional aide, who was not authorized to comment publicly and spoke on the condition of anonymity.

Maduro says the boat strikes are an attempt to undercut his authority and try to foment unrest that would lead to his ouster from power.

The State Department pushed back against the notion that the administration had been involved in anything other than an operation targeting drug traffickers.

"Maduro is not the legitimate leader of Venezuela; he`s a fugitive of American justice who undermines regional security and poisons Americans and we want to see him brought to justice," said Tommy Pigott, a State Department spokesman. "The U.S. is engaged in a counter-drug cartel operation and any claim that we are coordinating with anyone on anything other than this targeted effort is completely false."