The brothers accused of staging the Boston Marathon bombings planned a new attack in Times Square as they were being hunted by police, New York Mayor Michael Bloomberg said.

Dzhokhar Tsarnaev confessed the plan to Federal Bureau of Investigation (FBI) agents in his hospital bed, Bloomberg said, calling it a "horrific reminder that we remain targets for terrorists."

"Last night we were informed by the FBI that the surviving attacker revealed that New York City was next on their list of targets," Bloomberg told a press conference.

The pair -- accused of carrying out the April 15 attacks near the Boston Marathon finish line that killed three people and wounded 264 -- aimed to use their remaining explosives in Times Square, he added.

The 19-year-old and his elder brother Tamerlan Tsarnaev made a "spontaneous" decision to go to New York as they raced around the Boston suburbs in a car with at least six bombs, New York police chief Ray Kelly added.

The pair had a pressure cooker bomb -- similar to the two that erupted at the marathon -- as well as five other pipe bombs and improvised hand grenades as they drove around in the hijacked black Mercedes, according to police.

The night finished with a policeman killed and another gravely wounded. Tamerlan Tsarnaev, 26, was killed in one shootout with police while Dzhokhar, critically wounded, was found Friday hiding in a boat in a suburban backyard.

Kelly revealed on Wednesday that Dzhokhar Tsarnaev had told investigators that the pair planned to go to New York "to party."

"A subsequent questioning of Dzhokhar revealed that he and his brother decided spontaneously on Times Square as a target," Kelly said Thursday at a joint news conference with Bloomberg.

"They would drive to Times Square that same night. They discussed this while driving around in a Mercedes SUV that they had hijacked after they shot and killed an MIT police officer," said Kelly.

New York police have bolstered security in Times Square since a 2010 attempt to set off a car bomb was foiled.

The brothers' plan "fell apart when they realized that the vehicle that they hijacked was low on gas and ordered the driver to stop at a nearby gas station. The driver used the opportunity to escape and call the police," Kelly said.

The call by the driver of the hijacked car unleashed a manhunt involving thousands of police.

Dzhokhar Tsarnaev has a bullet wound to his throat but is in "fair" condition and is reported to have communicated with investigators.

He has been charged with using a weapon of mass destruction and could face the death penalty if convicted in US federal court.

The new revelations emerged after the mother of the two men, Zubeidat Tsarnaeva, accused US authorities of needlessly killing her son.

She expressed regret that the family ever moved to the United States.

Tamerlan was "killed, cruelly killed. I want to scream to the whole world, what did you do? He was alive!" Tsarnaeva, dressed in an Islamic headscarf, said in Makhachkala, the main city in the Russian region of Dagestan.

"He was alive! Why did they need to kill him? Why? They got him alive," she said, reaffirming her belief that her sons were innocent of the Boston bombings. "I know one thing, that my children did not do this."

She tearfully expressed regret over the family's move to the United States in 2002, waving her hands and banging the table with emotion.

"Why did I ever go there? Why? I thought that America will protect us and our kids, it will be safe," she said.

"But America took my kids away from me. Why would I not regret?"

Tsarnaeva and her estranged husband Anzor Tsarnaev met US investigators in Dagestan this week in a visit organized by the US embassy and the Russian authorities.

The father said he planned to leave for the United States on Friday.

There have been mounting questions in the United States about whether the US authorities missed crucial signals, about Tamerlan in particular, that should have raised suspicions about the brothers before the bombings.