SAN FRANCSICO -- Alaska Governor Sarah Palin, the Republican vice presidential nominee, cites vigilance against Russian warplanes coming into U.S. airspace over her state as one of her foreign policy credentials. But the U.S. military command in charge says that has not happened in her 21 months in office.

"When you consider even national security issues with Russia, as [Prime Minister Vladimir] Putin rears his head and comes into the airspace of the United States of America, where -- where do they go? It's Alaska," Palin said in an interview last week with CBS News' Katie Couric.

The spokeswoman for the John McCain-Palin campaign, Maria Comella, clarified in an e-mail that when "Russian incursions near Alaskan airspace and inside the air defense identification zone have occurred ... U.S. Air Force fighters have been scrambled repeatedly."

The air defense identification zone, almost completely over water, extends 19 kilometers past the perimeter of the United States. Most nations have similar areas.

However, no Russian military planes have been flying even into that zone, said Major Allen Herritage, a spokesman for the Alaska region of the North American Aerospace Defense Command. "To be very clear, there has not been any incursion in U.S. airspace in recent years," Herritage said.

What Palin might have been referring to was a buffer zone of airspace that extends beyond the 19-kilometer strip. Although not recognized internationally as the United States' to protect, the military watches it. That zone is where there have been increased exercises -- about 20 incidents in the last two years.

Asked about Herritage's statement, Palin's foreign policy adviser, Steve Biegun, said the candidate's position was correct. Russia's "old behaviors" of aggressively flying into U.S. airspace have been exhibited recently, he said.

Herritage said U.S. Air Force officials discussed with Palin instances of Russian planes entering the buffer zone and the U.S. response during their annual statehouse briefing in February.