By Edith Honan

NEW YORK (Reuters) - One World Trade Center, being built at the site of the fallen twin towers, surpassed the Empire State Building on Monday as the tallest building in New York.

Construction crews set in place a steel horizontal beam at a height of about 1,270 feet, topping by about 20 feet (six meters) the rooftop of the observation deck of the Empire State Building, which stands about 3 miles to the north in Midtown Manhattan.

Including the antenna tower, however, the iconic Empire State Building is still higher.

The Empire State Building, built in 1931, was the city's tallest at a height of 1,545 feet to the tip of its broadcast antenna until 1972 when it was overtaken by the original World Trade Center towers. It then regained the title after the September 11, 2001, attacks, which destroyed the complex.

Construction started six years ago on the new World Trade Center and now the skyscraper, formerly called the Freedom Tower, surpasses the top floor of the Empire State Building, Port Authority officials told reporters.

"The new World Trade Center is more than just a skyscraper: it is a symbol of the enduring spirit of the City and State of New York, representing our commitment to rebuilding stronger than before," New York Governor Andrew Cuomo said in commemorating the milestone.

One World Trade Center will stand at 1,776 feet to the tip of its antenna when it is completed, possibly by late 2013. Then it will top the Empire State entirely.

The skyscraper, only 55 percent of which is leased, will be higher than the former twin towers, which were toppled in the 2001 attacks when nearly 3,000 people were killed. The north tower stood 1,727 feet including its antenna.

(Reporting by Edith Honan; Editing by Philip Barbara)