Iraq could be planning a chemical or biological attack on American cities through the use of remote-controlled "drone" planes equipped with GPS tracking maps, according to U.S. intelligence.

The information about Iraq's unmanned aerial vehicle (UAV) program has caused a "real concern" among defense personnel, senior U.S. officials tell Fox News. They're worried that these vehicles have already been, or could be, transported inside the United States to be used in an attack, although there is no proof that this has happened.

Secretary of State Colin Powell showed a picture of a small drone plane during his presentation to the U.N. Security Council earlier this month.

"UAVs outfitted with spray tanks constitute an ideal method for launching a terrorist attack using biological weapons," Powell said during his speech. "Iraq could use these small UAVs, which have a wingspan of only a few meters, to deliver biological agents to its neighbors or, if transported, to other countries, including the United States."

Powell said there is "ample evidence" that Iraq has dedicated much time and effort to developing and testing spray devices that could be adapted for UAVs. "And of the little that Saddam Hussein told us about UAVs, he has not told the truth," Powell said.

In the arms declaration Iraq submitted to the U.N. Security Council in December, the country said its UAVs have a range of only 50 miles. But Powell said U.S. intelligence sources found that one of Iraq's newest UAVs went 310 miles nonstop on autopilot in a test run. That distance is over the 155 miles that the United Nations permits, and the test was left out of Iraq's arms declaration.

Officials tell Fox that there is solid intelligence that Iraq has tested many different types of sprayers on these drones to disperse chemical and biological weapons.

President Bush addressed the threat in October in Cincinnati, making his first big case outlining Iraq's defiance.

"We've also discovered through intelligence that Iraq has a growing fleet of manned and unmanned aerial vehicles that could be used to disperse chemical and biological weapons across broad areas," Bush said in preparation for a congressional vote authorizing the use of force against Iraq. "We're concerned that Iraq is exploring ways of using these UAVs for missions targeting the United States."

The president noted, however, that sophisticated delivery systems aren't required for a chemical or biological attack. "All that might be required are a small container and one terrorist or Iraqi intelligence operative to deliver it," he said.

Even though it has been mentioned a few times by administration officials, the issue of UAVs and their capabilities has been largely overlooked.

But some experts say that even if the UAVs do get assembled for use in the United States, the chances that they could cause widespread damage are low.

"These technologies are not terribly well proven," F. Whitten Peters, a former Air Force Secretary, told Fox News, referring to vehicles that can be used to disperse harmful agents.

Peters said in order to go undetected in the air, the UAVs would have to be small -- and therefore would not be able to carry too much of a harmful substance, and they would have to fly over densely populated areas if they want to achieve maximum casualties.

But because many large metropolitan areas such as Washington have air traffic watchers keeping an eye out for any nearby planes that have not filed a flight plan, the UAVs likely would not succeed in a large-city attack.

It's the smaller cities and towns that would be more vulnerable.

"It's not clear air traffic would actually see this aircraft," Peters said, adding that if the vehicles flew low enough to evade radar detection, "they would be basically invisible."

As to what the government could do to protect Americans from any threat UAVs may pose, Peters said: "I don't think there's much to be done besides the steps we're already taking to deal with chemical and biological threats."

But some experts say the threat is very real and should be taken seriously.

"This isn't brain surgery," Air National Guard Chief Paul Weaver told Fox News in reference to how easy it would be to assemble a UAV. "The key is getting it into the country."

Not too long after Sept. 11, there was a report made public about Usama bin Laden's Al Qaeda network being trained to conduct air raids through air vehicles outfitted with spray tanks. Some terror network members had looked into the possibility of training on the aerial UAVs. This was the catalyst for investigations into U.S. flight schools.

"If they could organize something like Sept. 11," Weaver said, "this would be very doable."

Fox News' Bret Baier and Liza Porteus contributed to this report.