SEAL Team 6, the special operations unit that took down Osama bin Laden, has emerged as "a global manhunting machine" whose secrecy has raised questions about its accountability, the New York Times reported Sunday.

The Times documented the SEALs growth in size and importance since 9/11, carrying out thousands of raids credited with weakening militant networks in places like Afghanistan and Somalia.

They fight alongside CIA paramilitary operatives, operate from spy ships disguised as commercial vessels, and shadow targets as undercover agents in US embassies, according to the Times, in an account based on interviews with dozens of former and current team members.

"Once a small group reserved for specialized but rare missions, the unit best known for killing Osama bin Laden has been transformed by more than a decade of combat into a global manhunting machine," the Times said.

For all its notoriety, however, it also has remained the most secretive and least scrutinized organization in the US military, and its activities have raised concerns about excessive killing and civilian deaths, the Times said.

In a 2012 incident in Afghanistan, an American doctor who was rescued from his Taliban captors, while grateful to be freed, told the Times that one of the kidnappers appeared to have been killed after surviving the raid.

All five captors were killed in the raid, as was the first of the SEALs to enter the compound where doctor Dilip Joseph was being held.

-- Coming to terms --

"It took me weeks to come to terms with the efficiency of the rescue," Joseph said. "It was so surgical."

A daring nighttime raid to rescue kidnapped British aid worker Linda Norgrove in Afghanistan ended in her death by a team member who hurled a grenade at what he thought were kidnappers.

Other rescues were accomplished with stunning success, such as that of US aid worker Jessica Buchanan and Danish colleague Poul Hagen, in Somalia.

The Times said the American operators sky-dived into Somalia, and crept up on their kidnappers under cover of darkness, shooting and killing all nine captors.

"Until they identified themselves, I did not believe a rescue was possible," Buchanan told the Times.

The Times identifies 2006 as a key point in the SEALs evolution, when lieutenant general Stanley McChrystal ordered the special operations forces to take a more expansive role in beating back a resurgent Taliban.

Team 6 was designated to lead the Special Operations force, embarking on nightly raids and intensifying but increasingly routine combat.

For weeks at a time between 2006 and 2008, units were logging 10 to 15 kills a night, sometimes as many as 25, according to the Times.

"These killing fests had become routine," a former Team 6 officer told the newspaper.

Another former top officer acknowledged "bad things went on."

"Do I think there was more killing than should have been done? Sure."

"I think the natural inclination was, if it's a threat, kill it, and later on you realize, 'Oh, maybe I overassessed the threat,' " he said.