Donald Trump has called for the suspect in the New York terror attack to be put to death, raising fears the intervention could impact the trial.

Federal prosecutors brought terrorism charges on Wednesday against Sayfullo Saipov, the Uzbek immigrant accused in the truck rampage that left eight people dead.

Handcuffed and with his feet shackled, Saipov appeared in court in a wheelchair after being shot by police before his arrest. The charges against the 29-year-old could bring the death penalty.

Having allegedly been inspired by jihadist propaganda online, Saipov even asked to display the flag of Islamic State of Iraq and the Levant in his hospital room room and said "he felt good about what he had done," prosecutors said in court papers.

"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!" the president tweeted.