US President Donald Trump has been accused of hindering the case against Sayfullo Saipov after twice angrily declaring the suspected New York terrorist should receive the death penalty.

Uzbek-born immigrant Saipov has been charged with terror offences after allegedly killing eight people when he drove a truck down a central New York City bike path.

Mr Trump stepped back from earlier threats to send Saipov to the notorious Guantanamo Bay prison in Cuba overnight, only to then call for his execution.

“Would love to send the NYC terrorist to Guantanamo but statistically that process takes much longer than going through the Federal system,” Trump wrote in a pair of posts on Twitter on Thursday.

“There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!”

…There is also something appropriate about keeping him in the home of the horrible crime he committed. Should move fast. DEATH PENALTY!

— Donald J. Trump (@realDonaldTrump) November 2, 2017

Earlier, he tweeted: “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!”

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!

— Donald J. Trump (@realDonaldTrump) November 2, 2017

Saipov, 29, told investigators he was inspired by Islamic State videos and began planning Tuesday’s attack a year ago, according to a criminal complaint filed Thursday.

He said “he felt good about what he had done” and asked for permission to display the IS flag in his hospital room.

The charges are one count of violence and destruction of motor vehicles causing the deaths of eight people and one count of providing material support and resources to a foreign terrorist organisation.

The maximum penalty for the first is death; the maximum for the second life in prison, Kim said.

Handcuffed, with his legs shackled and in a wheelchair, Saipov nodded his head repeatedly on Thursday as he was read his rights in a brief court proceeding that he followed through a Russian interpreter.

He was ordered to be held without bail.

Prosecutors said Saipov had 90 videos and 3800 photos on one of his two mobile phones, many of them IS-related pieces of propaganda, including images of prisoners being beheaded, shot or run over by a tank.

The charging document said Saipov was particularly motivated by a video in which IS leader Abu Bakr al-Baghdadi calls on Muslims in the US and elsewhere to support the group’s cause.

The Federal Bureau of Investigation said it had located another Uzbek man, Mukhammadzoir Kadirov, 32, as a person of interest in the attack.

Wednesday’s terror attack was the deadliest in New York since the September 11, 2001, when more than 2600 people died in the World Trade Centre attack.

Five Argentine tourists, a Belgian, a New Yorker and a New Jersey man were killed in the attack.

– With AAP