US to deploy 'bigger than ever' security for UNGA

New York city will also see an increased bag search and canine activities throughout the mass transit systems

New York City will deploy "bigger than ever" police presence during the upcoming UN General Assembly week after an explosion injured 29 on Saturday evening, Mayor Bill de Blasio said on Sunday.

"You will see a very substantial NYPD presence this week, bigger than ever," de Blasio told reporters at a press conference on the explosion, Xinhua reported.

Police officers will be deployed in force in key public locations around the United Nations headquarters and the Times Square, including the newer units of Critical Response Command and Strategic Response Group, said de Blasio.

The city will also see an increased bag search and canine activities throughout the mass transit systems, said city officials.

At the press conference, the mayor said all 29 people wounded in Saturday's blast were released from hospitals and the motivation behind the explosion remains unknown.

He termed the Saturday blast "a very serious incident", saying a lot more work needs to be done to find out the motivation behind the blast.

He called the blast an "intentional" and "criminal" act. However, he declined to call the blast a terrorist attack. "We'll not jump to conclusions," he said.

A second explosive device was found a few blocks near the explosion site.

The mayor noted there was no specific and credible threat against the city, calling for the vigilance of New Yorkers and asked for tips and information from the public.

He also said there was no specific evidence of connection between the New York explosion and a New Jersey pipe bomb blast, which happened on Saturday along the route of a Marines charity run, causing no injuries or damage.

Meanwhile, New York City Police Commissioner James O'Neill said at the press conference that no one has claimed responsibility for the New York explosion, which he categorised as a "violent criminal act".

City officials confirmed that the explosion in the popular Chelsea neighbourhood was caused by a bomb with "components indicative of an IED", not an accident.

New York police are beefing up extra security at bus terminals, airports and on subways in the city for the UN General Assembly week.

In an earlier press briefing, New York State Governor Andrew Cuomo also said the state will be deploying an additional 1,000 state troopers and members of the National Guard throughout New York City.

The suspects and motives were still unknown, and investigations were still in early stages, according to city officials.