NEW YORK (AFP) - Thieves inadvertently helped police find a bomb in New York's fashionable neighborhood of Chelsea, it emerged on Monday after the suspect wanted for planting the device and detonating another was arrested.

The pressure cooker device was discovered on 27th Street late Saturday - four blocks away from the site where a similar bomb exploded, wounding 29 people.

"We have a video of two persons who picked up the bag, took the device out of it and then walked off with the bag," New York police chief James O'Neill told a news conference on Monday.

"They look like there were two gentlemen just strolling up and down Seventh Avenue at the time," O'Neill added. "Once they picked up the bag, they seemed incredulous, picked it up off the street and walked off with it." Police are now urgently trying to track down and speak to the two witnesses.

It was "difficult to say right now" whether the thieves inadvertently pulled a wire on the device, which may have disabled the bomb, O'Neill said.

But a police spokesman told AFP they did not disarm the device, which was later safely defused by law enforcement and sent to the FBI for analysis.

In the neighbouring state of New Jersey, two homeless men also found the backpack that contained five pipe bombs overnight in Elizabeth, the hometown of Afghan-born suspected bomber Ahmad Khan Rahami.

The men picked the bag up hoping it might contain something valuable and walked down the street before realising there was something wrong, Elizabeth Mayor Chris Bollwage told reporters.

The homeless men found the package going through a garbage can, Bollwage was quoted as saying. But when they noticed wires and a pipe, they dropped the bag and went straight to the police.

Officers later disarmed the nest of bombs, accidentally detonating one of the devices in the process, but causing no injuries, the FBI said.

Police captured Rahami in New Jersey on Monday after shooting him in the leg during a shootout. He is wanted in connection with the New York bombing and a separate pipe bomb attack on the Jersey shore last Saturday.