By Gina Cherelus

NEW YORK (Reuters) - New York City will deploy its first class of police dogs trained to trace vapor trails left behind by would-be bombers as part of stepped-up security measures to protect the country's largest city during the Fourth of July weekend.

The so-called Vapor Wake Detection Dogs are capable of detecting air-borne scents from explosives even after the carrier has left the area, a skill that sets them apart from other bomb-detection canines.

The new canine patrols will join thousands of police officers, including the city's new counter-terrorism squad, in patrolling crowds gathered for a massive fireworks display over the East River and other Independence Day events.

The enhanced security comes at a time of heightened U.S. vigilance against potential attacks on vulnerable targets, including airports, concerts and clubs.

The long holiday weekend follows an attack on a Dhaka, Bangladesh cafe on Friday claimed by Islamic State and suicide bombings at Istanbul's main airport on Tuesday blamed on the militant group. A gunman inspired by Islamic State went on a shooting rampage in an Orlando gay nightclub on June 12.

An initial class of eight Labrador Retrievers, part of the NYPD'S Critical Response Command (CRC), graduated from the training program in March, two days after a series of coordinated bomb attacks in Belgium.

"This time last year we were just creating the CRC - the strategic response group - and beginning to acquire the vapor wake dogs," Police Commissioner Bill Bratton told reporters on Friday. "This year they are here, fully trained, fully equipped."

In preparation for the events on the Fourth of July, the NYPD will deploy the dogs and their handlers in undisclosed locations around the city.

The dogs can sniff out specific scent particles produced by the body's natural heat that are left behind by an individual wearing an explosive vest or carrying a bomb in a backpack, according to AMK9, the company that developed the program. That skill can enable police to track down the source of the vapors, hopefully in time to stop detonation.

Hoksand Lau, an NYPD detective with the CRC, said the ability of the dogs to detect vapor wakes should prove to be especially valuable in areas such as Times Square that are packed with pedestrians.

"I think the vapor wake will be very essential to the department, where we will be able to fill in any type of security gap," Lau said during the March graduation ceremony.

(Reporting by Gina Cherelus; Editing by Frank McGurty and Andrew Hay)