WASHINGTON (Reuters) - The skies above New Year's Eve revelers in New York, Las Vegas and some other U.S. cities will be off-limits to certain aircraft and patrolled by warplanes as part of increased vigilance for terror attacks, the Department of Homeland Security said on Tuesday.

Nine days after raising the nation's terror alert to its second highest level due to heightened fear of attack, the government said temporary flight restrictions, or TFRs, would be in place over Las Vegas, New York City, Washington and downtown Chicago.

"We granted requests for TFRs ... over (some) areas for New Year's Eve," department spokeswoman Rachael Sunbarger said.

The temporary flight restrictions, which apply to chartered flights and smaller leisure aircraft, but not to commercial flights, pertain to Las Vegas and New York City specifically during the New Year's holiday.

New York City's Times Square and the Strip in Las Vegas are two of the more popular sites for New Year's Eve celebrations, as hundreds of thousands of people cram into the area to ring in the New Year.

Homeland Security Secretary Tom Ridge has said the government would beef up security around large gatherings which are viewed as potential terror targets.

Flights over downtown Chicago have been restricted under a "code orange TFR" since Dec. 24, and there have been restrictions over Washington's airspace in place since the Sept. 11, 2001, attacks.

Flight restrictions will also be implemented for most holiday football bowl games, including the Rose Bowl in Pasadena, California, on New Year's Day and the giant parade that precedes it, Sunbarger said.

New York city officials, who expect about 750,000 people to gather around Times Square on New Year's Eve, said this would be one of the most heavily guarded celebrations ever.

"We know that New York remains on top of the terrorist target list and so we have to remain vigilant," Police Commissioner Raymond Kelly told reporters in New York.

The government has routinely used flight restrictions since the 2001 attacks, especially in areas President Bush is visiting. Restrictions are often put in place during major sporting events or at times of heightened security alert over places like Disneyland where large crowds gather.

MILITARY PATROLS

The United States blames Osama bin Laden's al Qaeda organization for the deaths of nearly 3,000 people on Sept. 11, 2001, when hijacked planes slammed into the World Trade Center, the Pentagon and a field in Pennsylvania.

Ridge, who announced on Monday his department had ordered foreign airlines to put armed marshals on selected flights to and from the United States, said military aircraft would also patrol U.S. skies.

"We will have in play ... not only temporary flight restrictions over cities but we will have aviation patrolling the skies and on alert," Ridge said on CBS's "Early Show."

Warplanes are often on patrol or on standby in key areas of the country, like the eastern corridor from New York to Washington, when the alert level is raised to orange. Ridge said on Monday planes would be ready to respond almost instantaneously if needed.

Homeland Security, which on Dec. 21 raised its terror alert to the second highest level -- orange or a "high" risk of attack -- has said it is concerned about al Qaeda's desire to attack commercial airlines in the United States.