NEW YORK (CNN/Money) - The heightened terror alert costs U.S. cities millions of dollars, and for many of them, that is money they can ill afford.

The cost of orange How one major city deals with the threat of terrorism and the costs it brings. (more)

President Bush raised the nation's threat level Tuesday to orange, or "high" -- the second-highest level -- after intelligence reports suggested al Qaeda may be getting more active and mount another attack within the United States.

Cities nationwide spend about $70 million a week on additional homeland security during periods of high alerts, the U.S. Conference of Mayors said in a recent survey.

The report said New York City alone spends about $5 million a week on additional homeland measures, and most of that is to keep police officers on 12-hour shifts and put extra law enforcement on duty.

San Francisco spends $2.6 million a week on heightened security, including protecting landmarks like the Golden Gate Bridge, the study said.

Many cities are facing their worst budget shortfalls in decades and they are struggling to meet these increased financial demands.

Some city officials have questioned whether the "high" alerts would do anything more than frighten the public and cost taxpayer dollars, but the White House maintained that the security alerts may help deter attacks.