Those planes that look like specks in the stratosphere are flying so high because they are merely passing by the United States - flying bananas to Germany, Canadians to Mexico and Europeans to Jamaica. But should that exempt such flights from the full security screening they would get if their destination were in this country?

As the Obama administration works to harden domestic defenses against terrorism, some experts point to a potential vulnerability from thousands of flights that pass over the United States each week.

Although the United States regulates overflights, the cargo aboard them is not screened to federal standards and passenger lists are not matched to names on the terrorist watch list maintained by the Transportation Security Administration.

The TSA says other countries "have their own cargo security protocols that apply to those aircraft." The TSA has not implemented the new Secure Flight program to scrutinize passengers boarding overflights. That behind-the-scenes operation is designed to ferret out potential terrorists through a process that begins with airlines collecting detailed information when someone buys a ticket.

Security experts are divided about the severity of the risk.

Scanning all the cargo that flies over the country "is totally unrealistic," said Yossi Sheffi, director of the Center for Transportation and Logistics at Massachusetts Institute of Technology. "We have tens of millions of packages flying almost every night. We can't stop the huge flow of packages from all over the world. There has to be a balance between acceptable risk and the economy."

But Richard Bloom, a longtime U.S. intelligence operative who teaches counterterroism courses at Embry-Riddle Aeronautical University in Arizona, said a terrorist could "explode a plane with a dirty bomb or a biological weapon or an actual nuclear weapon on board, and that material will spread wherever it crashes."

TSA spokesman Greg Soule said in a statement that the agency "continues to work with our international and industry partners to ensure the successful implementation of vetting overflights. Secure Flight is a phased-in program, and addressing routes that overfly the United States is the next phase in its implementation."

Some of the costs of implementing those programs would fall on other nations and their air carriers.

The TSA has authority to divert planes from U.S. airspace if it detects a security risk, and there have been occasions when planes have been turned away. The agency, however, declined to comment for security reasons.

The issue of cargo aboard international flights came to the fore in October when bombs packaged in printer cartridges were found aboard U.S.-bound cargo planes near London and in Dubai. According to U.S. and British officials, the packages sent from Yemen were addressed to Chicago-area synagogues and designed to detonate in flight.

The federal government estimates that 55 to 65 percent of cargo bound for the United States aboard passenger planes is screened. Many experts say that scanning the rest of it, and the vast volume flown in cargo planes, would be economically infeasible.

The former security chief at Tel Aviv's airport pointed out that the printer cartridge bombs evaded X-ray detection even after authorities knew they were in the packages.

"Congress would make a mistake by passing a requirement for 100 percent screening of cargo," said Rafi Ron, now a security consultant based in McLean. "What's the use of legislating 100 percent screening even if the bomb which triggered this legislation would not have been detected by it?"

The overflight planes cross the United States at an altitude so high they often appear no bigger than the dot in the exclamation point of a long vapor trail. Airlines pay the Federal Aviation Administration $33.72 per hundred nautical miles flown over or within 100 miles of the United States, and the flights are subject to the same stringent regulations established for all other planes since the Sept. 11, 2001, attacks.

The planes are required to have hardened cockpit doors to deter hijackers, crew movement is restricted and pilots must maintain contact with U.S. air traffic control centers. Even though they don't intend to land here, pilots must file flight plans and air traffic control centers monitor the course and altitude of the planes.

Should a plane stray off course or otherwise arouse suspicion, air traffic controllers are linked to the FAA emergency network that would spread an alarm, ultimately scrambling fighter jets to intercept the flight if necessary.

Most passenger plane overflights originate in or are bound for Canada, but the route over the North Pole is the most direct between northern Europe and the Americas. The FAA said about 92 percent of overflights by cargo planes were headed to or from Germany.

"There is an unfounded fear out there that other countries don't have security standards on a par with the United States," said Steven Lott, communications director for the International Air Transport Association, the trade group for 230 airlines that account for more than 90 percent of international passenger flights. "It's a myth to say these overflights are the Wild West."

The FAA recorded 307,000 overflights in 2009, a number that is both accurate and misleading in the context of the terrorist risk. It's based on billing records of all flights handled by U.S. air traffic controllers.

Many of those were trans-Pacific flights handled by U.S. controllers based in Guam. The flights never came within striking distance of the continental United States. Similarly, controllers in Puerto Rico direct flights bound for Central and South America, and U.S. controllers handle polar flights that skirt Alaska, bound from the Middle East and Asia.

Although the FAA could not break down the numbers to say how many planes fly over the continental United States, Lott estimates that it is between 2,000 and 3,000 each week.

"You could probably count on one hand the number of planes that are coming [over the U.S.] in any given month from countries that are considered [terrorist] hot spots," Lott said. "Overflights, to us, remain a pretty small risk. If you and I can go on the Internet and track a plane as it crosses the United States, you can be sure that the government can, too."