Security is being stepped up in New York after the city received a "credible" threat to its subway system.
Mayor Michael Bloomberg said the move followed a "specific threat" of a terrorist attack.
He warned residents to be extra vigilent and to report suspicious packages to police.
"The FBI recently shared with us a specific threat to our subway system," he said.
"We have done and will continue to do everything we can to protect this city. We will spare no
resource, we will spare no expensee.
"We have increased our police presence on our subway," he added.
Extra police officers, counter-terrorist units and dogs are to patrol the underground network.
Peter King, chairman of the House Homeland Security Committee, confirmed they had received "credible security threats" to the city's system.
"Obviously, this is a significant threat," he added.
New York has been on high alert for another attack since the September 11 attacks on the World Trade Centre.
More than seven million people travel on New York subways each day.
Source: Sky News