The mayor of New York City has promised a “very substantial” police presence in the wake of a bomb blast on Saturday night which injured 29 people.

Bill de Blasio was speaking as the city was put on terror alert ahead of world leaders arriving for a United Nation general assembly this week.

A second improvised device, made from a pressure cooker, was found four blocks away in the same fashionable Chelsea neighbourhood of Manhattan.

Police and the FBI were investigating whether there was a link to another explosion on Saturday, caused by a pipe bomb in New Jersey. That device was planted in a rubbish bin on the route of a charity race to benefit the US Marine Corps.

Islamic State claimed responsibility for a third incident, in a shopping centre in Minnesota, where a man stabbed nine people. He was shot dead on Saturday by an off-duty police officer.

The Foreign and Commonwealth Office has not altered its travel advice to New York but confirmed that an explosion occurred in the Chelsea neighbourhood causing a number of injuries. The area was sealed off to allow police to carry out investigations.

A thousand police and National Guard officers were reported as being deployed across New York last night, concentrated on transport hubs.

Mayor di Blasio said: “New Yorkers are very smart and very practical. They’re going to see a very substantial police presence – a well-armed, well-trained Critical Response group, Strategic Response Group – Critical Response Command, Strategic Response Group.

“You will see a lot of police presence, you will see it in the subway, you will see it at major events, you will see it at major crossroads of this city – well-armed police officers.

“For all New Yorkers, a central message we want to give today is be vigilant. Be vigilant at this point in time, not just because of this incident, be vigilant because we’re going into United Nations General Assembly week.

“We would normally have an expanded presence for the United Nations General Assembly. You will see an even stronger presence now.”

He added: “The investigatory agencies continue to look to see if there is any specific connection to the incident in New Jersey. At this point we do not have any specific evidence of a connection, but that will continue to be considered.”