A bombing injuring 29 people and the discovery of a second device blocks away in Manhattan, along with a blast in nearby New Jersey, prompted New York state officials to tighten security, just as the annual UN General Assembly debate ramps up this week.

The blast Saturday night in the hip Chelsea neighbourhood destroyed a garbage bin. The site is 3 kilometres from United Nations headquarters on the East River.

New York Mayor Bill de Blasio on Sunday vowed a "bigger than ever" police presence to protect the UN.

The US Homeland Security Department was "actively monitoring and participating in the investigations" of both the New York and New Jersey explosions, Homeland Security Secretary Jeh Johnson said.

He noted that the UN General Assembly debate, which begins Tuesday, was already designated a national special security event, for which thousands of federal personnel led by the Secret Service were deploying to the city.

"Any area in New York City associated with UNGA will therefore be subject to an extraordinarily high level of surveillance and security," Johnson said.

President Barack Obama was to meet today with Chinese Premier Li Keqiang and later Iraqi Prime Minister Haider al-Abadi at the Lotte New York Palace Hotel.

Obama was to visit the US mission outside the United Nations and spend the night in the city, before co-hosting a Tuesday summit on the global refugee crisis.

- dpa