ISIS will pay 'big price' for each attack on US: Trump

The US military has hit ISIS "much harder" over the past two days after the jihadist group claimed the attacker was one of their soldiers, Trump said.

"They will pay a big price for every attack on us!" Trump wrote on Twitter. The New York attack Tuesday left eight dead and 12 wounded.

Trump has said the alleged attacker, Uzbek immigrant Sayfullo Saipov, 29, should be executed.

Trump backed off initial comments that he would like to see the man sent to the US prison for terror suspects at Guantanamo Bay, Cuba.

Meanwhile, Donald Trump's decision to call for the death penalty for the suspect in this week's terrorist attack in New York could threaten his chance of a fair trial, legal experts have warned.

The president first called for the execution of Sayfullo Saipov on Wednesday and renewed his call on Thursday morning.

"NYC terrorist was happy as he asked to hang ISIS flag in his hospital room. He killed 8 people, badly injured 12. SHOULD GET DEATH PENALTY!" Trump tweeted on Wednesday night.

On Thursday morning he followed up by appearing to rule out sending Saipov to the Guantánamo Bay detention facility in Cuba, something he had mooted on Wednesday.

"Would love to send the NYC terrorist to Guantanamo but statistically that process takes much longer than going through the Federal system," the president tweeted. "There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!"

Saipov, 29, has been charged with causing the deaths of eight people and providing material support and resources to a foreign terrorist organization – Islamic State. He is accused of driving the truck down a bike path on Manhattan's west side, and was hospitalized after being shot by a police officer and arrested.

He told investigators he had been inspired by watching Islamic State videos and began planning Tuesday's attack a year ago, according to a criminal complaint filed against him on Wednesday.

"Making a broad statement that he deserves the death penalty before he has stood trial is something that absolutely influences jurors – this is coming from the most powerful person in the United States," said Anna Cominsky, a law professor at New York Law School.

She added: "What happened here was a horrible tragedy, but our constitution says no matter how egregious a crime, a defendant is innocent until proven guilty and in effect the president has already said the person is guilty. It's very disappointing and it's not how the system works. I think the legal community is surprised and it's this particular comment that's alarming."

Gannon Coens, a defence atorney at Bodiford Law in Tallahassee, Florida, said Trump was "a very divisive person".

"If I was defending this client I certainly would not be pleased. It does not help in any way and it would make my job more difficult." Coens said the comments might not make it impossible to pick a jury, but would probably slow the process down. The selection process, during which potential jurors are asked if they have heard or seen things that might make them biased in a case, can take weeks or even months in high profile cases. One legal expert, who asked not to be named, said: "Comments like this could pollute the jury pool. It was inappropriate." The chances of Saipov receiving the death penalty on federal charges in a court in New York are slim. The last time a federal case tried in New York ended in an execution was 1954.