President Trump has decided that the United States is engaged in a formal “armed conflict” with drug cartels his team has labeled terrorist organizations and that suspected smugglers for such groups are “unlawful combatants,” the administration said in a confidential notice to Congress this week.

The notice was sent to several congressional committees and obtained by The New York Times. It adds new detail to the administration’s thinly articulated legal rationale for why three U.S. military strikes the president ordered on boats in the Caribbean Sea last month, killing all 17 people aboard them, should be seen as lawful rather than murder.

Mr. Trump’s move to formally deem his campaign against drug cartels as an active armed conflict means he is cementing his claim to extraordinary wartime powers, legal specialists said. In an armed conflict, as defined by international law, a country can lawfully kill enemy fighters even when they pose no threat, detain them indefinitely without trials and prosecute them in military courts.

Geoffrey S. Corn, a retired judge advocate general lawyer who was formerly the Army’s senior adviser for law-of-war issues, said drug cartels were not engaged in “hostilities” — the standard for when there is an armed conflict for legal purposes — against the United States because selling a dangerous product is different than an armed attack.

Noting that it is illegal for the military to deliberately target civilians who are not directly participating in hostilities — even suspected criminals — Mr. Corn called the president’s move an “abuse” that crossed a major legal line.

“This is not stretching the envelope,” he said. “This is shredding it. This is tearing it apart.”

The White House did not respond to a request for comment.

The Trump administration had called those strikes self-defense, asserting that the targets were smuggling drugs for cartels that the administration has designated as terrorists and invoking the laws of war to justify killing them rather arresting them. The administration has also stressed that about 100,000 Americans annually die from overdoses.

However, the focus of the administration’s attacks has been boats from Venezuela. The surge of overdose deaths in recent years has been driven by fentanyl that drug trafficking experts say comes from Mexico, not South America. Beyond factual issues, the bare-bones argument has been broadly criticized on legal grounds by specialists in armed-conflict law.

The notice to Congress, which was deemed controlled but unclassified information, cites a statute requiring reports to lawmakers about hostilities involving U.S. armed forces. It repeats the administration’s earlier arguments but also goes further with new claims, including portraying the U.S. military’s attacks on boats to be part of a sustained, active conflict rather than isolated acts of claimed self-defense.

Specifically, it says that Mr. Trump has “determined” that cartels engaged in smuggling drugs are “nonstate armed groups” whose actions “constitute an armed attack against the United States.” And it cites a term from international law — a “noninternational armed conflict” — that refers to a war with a nonstate actor.

“Based upon the cumulative effects of these hostile acts against the citizens and interests of the United States and friendly foreign nations, the president determined that the United States is in a noninternational armed conflict with these designated terrorist organizations,” the notice said.

There are different kinds of wars, and the concept of a “noninternational armed conflict,” developed in 20th-century law to mean a civil war in one country, as opposed to a war between two or more nation-states.

After the terrorist attacks on Sept. 11, 2001, when the United States went to war against Al Qaeda — a nonstate actor operating across multiple countries — some legal scholars objected that the country was stretching the rules to justify using wartime powers against a group they likened more to a criminal band of pirates.

But the Supreme Court found that the conflict with Al Qaeda was a real war. It blessed as lawful the Bush administration’s use of the wartime power to hold captured Qaeda members in indefinite detention without trial, while also saying the government was bound by the Geneva Conventions to treat such prisoners humanely and not torture them.

The court’s reasoning, however, turned on the fact that Al Qaeda had attacked the United States using hijacked airplanes as weapons to intentionally kill people, and that Congress had authorized the use of armed force against it. Indeed, in a 2006 ruling, the court also rejected the Bush administration’s first attempt to use military commissions, saying that lawmakers needed to explicitly authorize their use.

In this case, the Trump administration is conflating the trafficking of an illicit and dangerous consumer product with a use of force and an armed attack. Congress has not authorized the use of any type of military force against cartels.

The U.S. government has routinely said it is engaged in a metaphorical “war on drugs,” meaning aggressive law enforcement. Mr. Trump’s claim that he can and has put the country into a literal state of war against drug cartels is important for legal reasons. Police arrest suspected drug dealers; it would be a crime to instead summarily gun them down. But in an armed conflict, it is lawful to kill combatants for the opposing force on sight.

The notice to Congress also justified the most recent publicly disclosed attack on a boat — in which U.S. Special Operations Forces killed all three people on Sept. 15 — by calling the crew “unlawful combatants,” as if they were soldiers on a battlefield.

“The vessel was assessed by the U.S. intelligence community to be affiliated with a designated terrorist organization and, at the time, engaged in trafficking illicit drugs, which could ultimately be used to kill Americans,” the notice said. “This strike resulted in the destruction of the vessel, the illicit narcotics, and the death of approximately three unlawful combatants.”

The notice to Congress did not specifically name any of the drug cartels with which Mr. Trump claims the United States is engaged in an armed conflict. It also did not specify any standards the administration is using to determine whether particular suspects have sufficient links to such groups for the military to kill them.

Brian Finucane, a former State Department lawyer who is a specialist in the laws of armed conflict and has criticized the boat attacks, expressed skepticism. Among other things, under international law, for a nonstate entity to be part of an armed conflict, it must meet the standard of being an organized armed group, he noted.

“Not surprised that the administration may have settled on such a theory to legally backfill their operations,” Mr. Finucane said. “I had speculated they might do so. One major problem, however, is that it is far from clear that whoever they are targeting is an organized armed group such that the U.S. could be in a N.I.A.C. with it,” he added, referring to a noninternational armed conflict.

Mr. Finucane questioned whether the gang the Trump administration has most talked about targeting, Tren de Aragua, is coherent enough to meet that threshold. An intelligence assessment in April, which was declassified and made public the following month, said the Venezuelan group consisted of “loosely organized cells of localized individual criminal networks” and was so “decentralized” that it would be difficult for it to coordinate actions.