At least 87 people died and 32 Iranian sailors were rescued after the sub torpedoed the IRIS Dena in international waters near Sri Lanka.
While the attacks on Iran overall are "a clear violation of the UN charter", the Dena was "a clear military target", said Marko Milanovic, professor of international law at the University of Reading in Britain.
"Targeting a military vessel is not a war crime," Milanovic said.
Australia's government confirmed on Friday three Australians were aboard the submarine that sank the IRIS Dena.
The Australians were there as part of the trilateral US, Australian and UK training program under the AUKUS security pact.
The IRIS Dena had just participated in naval exercises hosted by India and was heading into international waters on its way home, the Indian government said.
But neither its location far from the war zone nor its presence for the joint naval exercises affects the legality of the strike, said Rachel VanLandingham, a professor at Southwestern Law School and a retired lieutenant colonel in the US Air Force who served as a judge advocate general.
"It doesn't matter that it wasn't firing at the time," she said of the Iranian ship. "It matters that it can be used to fire at American military assets."
But what transpired after the torpedo struck the ship could raise concerns.
"The attack may not violate the law of war, but that's only the start of the analysis," said Brian Finucane, who served in the State Department's Office of the Legal Adviser for a decade. "What happens after the attack is another matter."
International humanitarian law says the US should have taken "all possible measures" to help search for and collect anyone who was wounded or shipwrecked.
US naval forces have the same obligation under the Pentagon's rules as laid out in the Defence Department Law of War Manual. But the manual notes "practical limitations", including the cramped quarters of a submarine, could require it to alert other ships, airplanes or nearby authorities of the location of possible survivors.
Bringing people inside a submarine - one of the most closely guarded platforms in the US Navy - also poses issues.
A US official, who spoke on condition of anonymity, confirmed that following the strike, the United States contacted Sri Lankan authorities to provide them with the ship's location for search and rescue efforts.
Sri Lanka's foreign minister, Vijitha Herath, told parliament his nation's navy received a distress signal from the stricken ship, and sent ships and planes on a rescue mission.
But by the time Sri Lanka's navy reached the location, there was no sign of the ship, "only some oil patches and life rafts", navy spokesman Commander Buddhika Sampath said. "We found people floating on the water".
