Republican Donald Trump has repeated calls for the return of waterboarding against Islamic State militants, saying: "I like it a lot."
His comments at a rally in Ohio came hours after suicide bombers killed 41 people at an airport in Istanbul.
"You have to fight fire with fire," said the Republicans' likely nominee, after referring to IS beheadings.
Waterboarding, described by President Barack Obama as torture, was banned by the US in 2006.
The Turkish authorities believe the so-called Islamic State was behind the attacks at Ataturk International Airport on Tuesday.
"We have to fight so viciously and violently because we're dealing with violent people," Mr Trump said.
At one point, he asked the crowd: "What do you think about waterboarding?"
They cheered as he gave his answer: "I like it a lot. I don't think it's tough enough."
The New York tycoon lamented that the US is prevented from waterboarding but "they [Islamic State] can do chopping off heads, drowning people in steel cages, they can do whatever they want to do".
Democratic presumptive nominee Hillary Clinton, who will face Mr Trump in November's election, said in response to the attacks in Istanbul that the US "cannot retreat".
"We must deepen our co-operation with our allies and partners in the Middle East and Europe to take on this threat".
Mr Trump has faced much criticism - even from within his own party - by calling on the US to close its borders to Muslim arrivals.
Recently there are signs that he is retreating from that and instead recommending a ban from people arriving from "terror nations" although it's not clear which countries they are.
Earlier on Tuesday, he compared the Trans-Pacific Partnership trade agreement to rape and threatened to confront China in a trade war.
He says that free trade, which has been Republican orthodoxy for many years, hurts American workers.